diff --git a/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregate.php b/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregate.php
new file mode 100644
index 0000000..53599db
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregate.php
@@ -0,0 +1,293 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\AssetAggregate.
+ */
+
+namespace Drupal\Core\Asset\Aggregate;
+
+use Assetic\Filter\FilterCollection;
+use Assetic\Filter\FilterInterface;
+use Drupal\Core\Asset\AssetInterface;
+use Assetic\Asset\AssetInterface as AsseticAssetInterface;
+use Drupal\Core\Asset\Aggregate\AssetAggregateInterface;
+use Drupal\Core\Asset\Collection\BasicAssetCollection;
+use Drupal\Core\Asset\Exception\AssetTypeMismatchException;
+use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
+use Drupal\Core\Asset\Metadata\AssetMetadataInterface;
+
+/**
+ * Base class for representing aggregate assets.
+ */
+class AssetAggregate extends BasicAssetCollection implements \IteratorAggregate, AssetInterface, AssetAggregateInterface {
+
+  /**
+   * @var \Drupal\Core\Asset\Metadata\AssetMetadataInterface
+   */
+  protected $metadata;
+
+  /**
+   * A string identifier for this aggregate.
+   *
+   * For how this is calculated, see:
+   * @see AssetAggregate::calculateId()
+   *
+   * @var string
+   */
+  protected $id;
+
+  /**
+   * The body of the aggregate asset. This is lazy-loaded.
+   *
+   * @var string
+   */
+  protected $content;
+
+  /**
+   * A collection of filters to be applied to this asset.
+   *
+   * @var FilterCollection
+   */
+  protected $filters;
+
+  /**
+   * The relative path to the asset.
+   *
+   * @var string
+   */
+  protected $sourcePath;
+
+  /**
+   * The desired path at which the asset should be dumped.
+   *
+   * @var string
+   */
+  protected $targetPath;
+
+  /**
+   * Internal state flag indicating whether or not load filters have been run.
+   *
+   * @var bool
+   */
+  protected $loaded = FALSE;
+
+  /**
+   * @param AssetMetadataInterface $metadata
+   *   The metadata bag for this aggregate.
+   * @param array $assets
+   *   Assets to add to this aggregate.
+   * @param array $filters
+   *   Filters to apply to this aggregate.
+   */
+  public function __construct(AssetMetadataInterface $metadata, $assets = array(), $filters = array()) {
+    $this->metadata = $metadata;
+    parent::__construct($assets);
+    $this->filters = new FilterCollection($filters);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function id() {
+    if (empty($this->id)) {
+      $this->calculateId();
+    }
+
+    return $this->id;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAssetType() {
+    return $this->metadata->getType();
+  }
+
+  /**
+   * Calculates and stores an id for this aggregate from the contained assets.
+   *
+   * @return void
+   */
+  protected function calculateId() {
+    $id = '';
+    foreach ($this->eachLeaf() as $asset) {
+      $id .= $asset->id();
+    }
+    // TODO come up with something stabler/more serialization friendly than object hash
+    $this->id = hash('sha256', $id ?: spl_object_hash($this));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMetadata() {
+    // TODO should this immutable? doable if we further granulate the interfaces
+    return $this->metadata;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeLeaf(AsseticAssetInterface $needle, $graceful = FALSE) {
+    if (!$needle instanceof AssetInterface) {
+      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset provided; Drupal aggregates require Drupal-flavored assets.');
+    }
+
+    return $this->doRemove($needle, $graceful);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function replaceLeaf(AsseticAssetInterface $needle, AsseticAssetInterface $replacement, $graceful = FALSE) {
+    if (!($needle instanceof AssetInterface && $replacement instanceof AssetInterface)) {
+      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset(s) provided; Drupal aggregates require Drupal-flavored assets.');
+    }
+
+    $this->ensureCorrectType($replacement);
+    if ($this->contains($replacement)) {
+      throw new \LogicException('Asset to be swapped in is already present in the collection.');
+    }
+
+    return $this->doReplace($needle, $replacement, $graceful);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function ensureCorrectType(AssetInterface $asset) {
+    if ($asset->getAssetType() != $this->getAssetType()) {
+      throw new AssetTypeMismatchException(sprintf('Aggregate/asset incompatibility, aggregate of type "%s", asset of type "%s". Aggregates and their contained assets must be of the same type.', $this->getAssetType(), $asset->getAssetType()));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * Aggregate assets are inherently eligible for preprocessing, so this is
+   * always true.
+   */
+  public function isPreprocessable() {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function load(FilterInterface $additionalFilter = NULL) {
+    // loop through leaves and load each asset
+    $parts = array();
+    foreach ($this as $asset) {
+      $asset->load($additionalFilter);
+      $parts[] = $asset->getContent();
+    }
+
+    $this->content = implode("\n", $parts);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function dump(FilterInterface $additionalFilter = NULL) {
+    // loop through leaves and dump each asset
+    $parts = array();
+    foreach ($this as $asset) {
+      $parts[] = $asset->dump($additionalFilter);
+    }
+
+    return implode("\n", $parts);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContent() {
+    return $this->content;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setContent($content) {
+    $this->content = $content;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLastModified() {
+    // TODO: Implement getLastModified() method.
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSourceRoot() {
+    // Drupal doesn't use this in general, and especially not for aggregates.
+    return NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSourcePath() {
+    return $this->sourcePath;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetPath() {
+    return $this->targetPath;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setTargetPath($targetPath) {
+    $this->targetPath = $targetPath;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function ensureFilter(FilterInterface $filter) {
+    $this->filters->ensure($filter);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFilters() {
+    $this->filters->all();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function clearFilters() {
+    $this->filters->clear();
+  }
+
+  /**
+   * @throws \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public final function getVars() {
+    throw new UnsupportedAsseticBehaviorException("Drupal does not use or support Assetic's 'vars' concept.");
+  }
+
+  /**
+   * @throws \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public final function setValues(array $values) {
+    throw new UnsupportedAsseticBehaviorException("Drupal does not use or support Assetic's 'values' concept.");
+  }
+
+  /**
+   * @throws \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public final function getValues() {
+    throw new UnsupportedAsseticBehaviorException("Drupal does not use or support Assetic's 'values' concept.");
+  }
+}
diff --git a/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregateInterface.php b/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregateInterface.php
index 7b40820..1a71540 100644
--- a/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregateInterface.php
+++ b/core/lib/Drupal/Core/Asset/Aggregate/AssetAggregateInterface.php
@@ -35,42 +35,4 @@
  */
 interface AssetAggregateInterface extends AssetInterface, AssetCollectionBasicInterface, AsseticAssetCollectionInterface {
 
-  /**
-   * Replaces an existing asset in the aggregate with a new one.
-   *
-   * This maintains ordering of the assets within the aggregate; the new asset
-   * will occupy the same position as the old asset.
-   *
-   * @param AssetInterface|string $needle
-   *   Either an AssetInterface instance, or the string id of an asset.
-   * @param AssetInterface $replacement
-   *   The new asset to swap into place.
-   * @param bool $graceful
-   *   Whether failure should return FALSE or throw an exception.
-   *
-   * @return bool
-   *
-   * @throws \OutOfBoundsException
-   */
-  public function replace($needle, AssetInterface $replacement, $graceful = FALSE);
-
-  /**
-   * Adds an asset to this aggregate.
-   *
-   * @param AsseticAssetInterface $asset
-   *   The asset to add. Note that, despite the type requirements, it must
-   *   conform to Drupal's AssetInterface.
-   *
-   * @return bool
-   *   TRUE if the asset was added successfully, FALSE if it was already present
-   *   in the aggregate.
-   *
-   * @throws UnsupportedAsseticBehaviorException
-   *   Thrown if a vanilla Assetic asset is provided.
-   *
-   * @throws AssetTypeMismatchException
-   *   Thrown if the provided asset is not the correct type for the aggregate
-   *   (e.g., CSS file in a JS aggregate).
-   */
-  public function add(AsseticAssetInterface $asset);
 }
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Asset/Aggregate/BaseAggregateAsset.php b/core/lib/Drupal/Core/Asset/Aggregate/BaseAggregateAsset.php
deleted file mode 100644
index c795a0e..0000000
--- a/core/lib/Drupal/Core/Asset/Aggregate/BaseAggregateAsset.php
+++ /dev/null
@@ -1,365 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Asset\BaseAggregateAsset.
- */
-
-namespace Drupal\Core\Asset\Aggregate;
-
-use Assetic\Filter\FilterCollection;
-use Assetic\Filter\FilterInterface;
-use Drupal\Core\Asset\Aggregate\Iterator\AssetAggregateIterator;
-use Drupal\Core\Asset\AsseticAdapterAsset;
-use Drupal\Core\Asset\AssetInterface;
-use Assetic\Asset\AssetInterface as AsseticAssetInterface;
-use Drupal\Core\Asset\Aggregate\AssetAggregateInterface;
-use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
-use Drupal\Core\Asset\Metadata\AssetMetadataInterface;
-
-/**
- * Base class for representing aggregate assets.
- *
- * TODO With PHP5.4, refactor out AssetCollectionBasicInterface into a trait.
- */
-abstract class BaseAggregateAsset extends AsseticAdapterAsset implements \IteratorAggregate, AssetInterface, AssetAggregateInterface {
-
-  /**
-   * @var \Drupal\Core\Asset\Metadata\AssetMetadataInterface
-   */
-  protected $metadata;
-
-  /**
-   * Container for all assets attached to this object.
-   *
-   * @var \SplObjectStorage
-   */
-  protected $assetStorage;
-
-  /**
-   * @var \SplObjectStorage
-   */
-  protected $nestedStorage;
-
-  /**
-   * A string identifier for this aggregate.
-   *
-   * This is calculated based on
-   *
-   * @var string
-   */
-  protected $id;
-
-  /**
-   * Maintains a map, keyed by id, of all assets.
-   *
-   * This map is also the canonical source for ordering information.
-   *
-   * @var array
-   */
-  protected $assetIdMap = array();
-
-  protected $content;
-
-  /**
-   * @param AssetMetadataInterface $metadata
-   *   The metadata bag for this aggregate.
-   * @param array $assets
-   *   Assets to add to this aggregate.
-   * @param array $filters
-   *   Filters to apply to this aggregate.
-   */
-  public function __construct(AssetMetadataInterface $metadata, $assets = array(), $filters = array()) {
-    parent::__construct($filters);
-
-    $this->metadata = $metadata;
-    $this->assetStorage = new \SplObjectStorage();
-    $this->nestedStorage = new \SplObjectStorage();
-
-    foreach ($assets as $asset) {
-      $this->add($asset);
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function id() {
-    if (empty($this->id)) {
-      $this->calculateId();
-    }
-
-    return $this->id;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getAssetType() {
-    return $this->metadata->getType();
-  }
-
-  /**
-   * Calculates and stores an id for this aggregate from the contained assets.
-   *
-   * @return void
-   */
-  protected function calculateId() {
-    $id = '';
-    foreach ($this as $asset) {
-      $id .= $asset->id();
-    }
-    // TODO come up with something stabler/more serialization friendly than object hash
-    $this->id = hash('sha256', $id ?: spl_object_hash($this));
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getMetadata() {
-    // TODO should this immutable? doable if we further granulate the interfaces
-    return $this->metadata;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function add(AsseticAssetInterface $asset) {
-    if (!$asset instanceof AssetInterface) {
-      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset provided; Drupal aggregates require Drupal-flavored assets.');
-    }
-    $this->ensureCorrectType($asset);
-
-    if ($this->contains($asset) || $this->getById($asset->id())) {
-      return FALSE;
-    }
-
-    $this->assetStorage->attach($asset);
-    $this->assetIdMap[$asset->id()] = $asset;
-
-    if ($asset instanceof AssetAggregateInterface) {
-      $this->nestedStorage->attach($asset);
-    }
-
-    return TRUE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function contains(AssetInterface $asset) {
-    if ($this->assetStorage->contains($asset)) {
-      return TRUE;
-    }
-
-    foreach ($this->nestedStorage as $aggregate) {
-      if ($aggregate->contains($asset)) {
-        return TRUE;
-      }
-    }
-
-    return FALSE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getById($id, $graceful = TRUE) {
-    if (isset($this->assetIdMap[$id])) {
-      return $this->assetIdMap[$id];
-    }
-    else {
-      // Recursively search for the id
-      foreach ($this->nestedStorage as $aggregate) {
-        if ($found = $aggregate->getById($id)) {
-          return $found;
-        }
-      }
-    }
-
-    if ($graceful) {
-      return FALSE;
-    }
-
-    throw new \OutOfBoundsException(sprintf('This aggregate does not contain an asset with id %s.', $id));
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function remove($needle, $graceful = TRUE) {
-    if (is_string($needle)) {
-      if (!$needle = $this->getById($needle, $graceful)) {
-        return FALSE;
-      }
-    }
-
-    return $this->removeLeaf($needle, $graceful);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function removeLeaf(AsseticAssetInterface $needle, $graceful = FALSE) {
-    if (!$needle instanceof AssetInterface) {
-      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset provided; Drupal aggregates require Drupal-flavored assets.');
-    }
-    $this->ensureCorrectType($needle);
-
-    foreach ($this->assetIdMap as $id => $asset) {
-      if ($asset === $needle) {
-        unset($this->assetStorage[$asset], $this->assetIdMap[$id], $this->nestedStorage[$asset]);
-
-        return TRUE;
-      }
-
-      if ($asset instanceof AssetAggregateInterface && $asset->removeLeaf($needle, $graceful)) {
-        return TRUE;
-      }
-    }
-
-    if ($graceful) {
-      return FALSE;
-    }
-
-    throw new \OutOfBoundsException('Asset not found.');
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function replace($needle, AssetInterface $replacement, $graceful = TRUE) {
-    if (is_string($needle)) {
-      if (!$needle = $this->getById($needle, $graceful)) {
-        return FALSE;
-      }
-    }
-
-    return $this->replaceLeaf($needle, $replacement, $graceful);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function replaceLeaf(AsseticAssetInterface $needle, AsseticAssetInterface $replacement, $graceful = FALSE) {
-    if (!($needle instanceof AssetInterface && $replacement instanceof AssetInterface)) {
-      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset(s) provided; Drupal aggregates require Drupal-flavored assets.');
-    }
-    $this->ensureCorrectType($needle);
-    $this->ensureCorrectType($replacement);
-
-    if ($this->contains($replacement)) {
-      throw new \LogicException('Asset provided for replacement is already present in the aggregate.');
-    }
-
-    $i = 0;
-    foreach ($this->assetIdMap as $id => $asset) {
-      if ($asset === $needle) {
-        unset($this->assetStorage[$asset], $this->nestedStorage[$asset]);
-
-        array_splice($this->assetIdMap, $i, 1, array($replacement->id() => $replacement));
-        $this->assetStorage->attach($replacement);
-        if ($replacement instanceof AssetAggregateInterface) {
-          $this->nestedStorage->attach($replacement);
-        }
-
-        return TRUE;
-      }
-
-      if ($asset instanceof AssetAggregateInterface && $asset->replaceLeaf($needle, $replacement, $graceful)) {
-        return TRUE;
-      }
-      $i++;
-    }
-
-    if ($graceful) {
-      return FALSE;
-    }
-
-    throw new \OutOfBoundsException('Asset not found.');
-  }
-
-  /**
-   * {@inheritdoc}
-   *
-   * Aggregate assets are inherently eligible for preprocessing, so this is
-   * always true.
-   */
-  public function isPreprocessable() {
-    return TRUE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function all() {
-    return $this->assetIdMap;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function load(FilterInterface $additionalFilter = NULL) {
-    // loop through leaves and load each asset
-    $parts = array();
-    foreach ($this as $asset) {
-      $asset->load($additionalFilter);
-      $parts[] = $asset->getContent();
-    }
-
-    $this->content = implode("\n", $parts);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function dump(FilterInterface $additionalFilter = NULL) {
-    // loop through leaves and dump each asset
-    $parts = array();
-    foreach ($this as $asset) {
-      $parts[] = $asset->dump($additionalFilter);
-    }
-
-    return implode("\n", $parts);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getContent() {
-    return $this->content;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setContent($content) {
-    $this->content = $content;
-  }
-
-  /**
-   * TODO Assetic uses their iterator to clone, then populate values and return here; is that a good model for us?
-   */
-  public function getIterator() {
-    return new \RecursiveIteratorIterator(new AssetAggregateIterator($this));
-  }
-
-  /**
-   * Indicates whether this collection contains any assets.
-   *
-   * @return bool
-   *   TRUE if contained assets are present, FALSE otherwise.
-   */
-  public function isEmpty() {
-    return $this->assetStorage->count() === 0;
-  }
-
-  /**
-   * Ensures that the asset is of the correct subtype (e.g., css vs. js).
-   *
-   * @param AssetInterface $asset
-   *
-   * @throws \Drupal\Core\Asset\Exception\AssetTypeMismatchException
-   */
-  abstract protected function ensureCorrectType(AssetInterface $asset);
-}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Asset/Aggregate/CssAggregateAsset.php b/core/lib/Drupal/Core/Asset/Aggregate/CssAggregateAsset.php
deleted file mode 100644
index 10ca555..0000000
--- a/core/lib/Drupal/Core/Asset/Aggregate/CssAggregateAsset.php
+++ /dev/null
@@ -1,40 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Asset\CssAggregateAsset.
- */
-
-namespace Drupal\Core\Asset\Aggregate;
-use Drupal\Core\Asset\AssetInterface;
-use Drupal\Core\Asset\Exception\AssetTypeMismatchException;
-use Drupal\Core\Asset\Metadata\AssetMetadataInterface;
-use Drupal\Core\Asset\Metadata\CssMetadataBag;
-
-/**
- * A CSS asset that is an aggregate of multiple other CSS assets.
- */
-class CssAggregateAsset extends BaseAggregateAsset {
-
-  /**
-   * {@inheritdoc}
-   *
-   * @throws \Drupal\Core\Asset\Exception\AssetTypeMismatchException
-   */
-  public function __construct(AssetMetadataInterface $metadata, $assets = array(), $filters = array(), $sourceRoot = array()) {
-    if (!$metadata instanceof CssMetadataBag) {
-      throw new AssetTypeMismatchException('CSS aggregates require CSS metadata bags.');
-    }
-
-    parent::__construct($metadata, $assets, $filters, $sourceRoot);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function ensureCorrectType(AssetInterface $asset) {
-    if ($asset->getAssetType() !== 'css') {
-      throw new AssetTypeMismatchException('CSS aggregates can only work with CSS assets.');
-    }
-  }
-}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Asset/Aggregate/Iterator/AssetAggregateIterator.php b/core/lib/Drupal/Core/Asset/Aggregate/Iterator/AssetAggregateIterator.php
deleted file mode 100644
index ed9eec1..0000000
--- a/core/lib/Drupal/Core/Asset/Aggregate/Iterator/AssetAggregateIterator.php
+++ /dev/null
@@ -1,23 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Asset\Aggregate\Iterator\AssetAggregateIterator.
- */
-
-namespace Drupal\Core\Asset\Aggregate\Iterator;
-
-use Drupal\Core\Asset\Aggregate\AssetAggregateInterface;
-
-/**
- * Iterates over an AssetAggregateInterface, returning each non-aggregate asset.
- */
-class AssetAggregateIterator extends \RecursiveArrayIterator {
-  public function __construct(AssetAggregateInterface $aggregate) {
-    parent::__construct($aggregate->all());
-  }
-
-  public function hasChildren() {
-    return $this->current() instanceof AssetAggregateInterface;
-  }
-}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Asset/Aggregate/JsAggregateAsset.php b/core/lib/Drupal/Core/Asset/Aggregate/JsAggregateAsset.php
deleted file mode 100644
index 2c9f86d..0000000
--- a/core/lib/Drupal/Core/Asset/Aggregate/JsAggregateAsset.php
+++ /dev/null
@@ -1,40 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Asset\Aggregate\JsAggregateAsset.
- */
-
-namespace Drupal\Core\Asset\Aggregate;
-use Drupal\Core\Asset\AssetInterface;
-use Drupal\Core\Asset\Exception\AssetTypeMismatchException;
-use Drupal\Core\Asset\Metadata\AssetMetadataInterface;
-use Drupal\Core\Asset\Metadata\JsMetadataBag;
-
-/**
- * A Javascript asset that aggregates together multiple other Javascript assets.
- */
-class JsAggregateAsset extends BaseAggregateAsset {
-
-  /**
-   * {@inheritdoc}
-   *
-   * @throws \Drupal\Core\Asset\Exception\AssetTypeMismatchException
-   */
-  public function __construct(AssetMetadataInterface $metadata, $assets = array(), $filters = array(), $sourceRoot = array()) {
-    if (!$metadata instanceof JsMetadataBag) {
-      throw new AssetTypeMismatchException('JS aggregates require JS metadata bags.');
-    }
-
-    parent::__construct($metadata, $assets, $filters, $sourceRoot);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function ensureCorrectType(AssetInterface $asset) {
-   if ($asset->getAssetType() !== 'js') {
-      throw new AssetTypeMismatchException('JS aggregates can only work with JS assets.');
-    }
-  }
-}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Asset/AsseticAdapterAsset.php b/core/lib/Drupal/Core/Asset/AsseticAdapterAsset.php
index de2980c..a1b28fc 100644
--- a/core/lib/Drupal/Core/Asset/AsseticAdapterAsset.php
+++ b/core/lib/Drupal/Core/Asset/AsseticAdapterAsset.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Asset;
 
-use Assetic\Asset\AssetInterface;
+use Assetic\Asset\AssetInterface as AsseticAssetInterface;
 use Assetic\Asset\BaseAsset as AsseticBaseAsset;
 use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
 
@@ -15,7 +15,7 @@
  * A class that reduces boilerplate code by centrally disabling the Assetic
  * properties and methods Drupal does not support.
  */
-abstract class AsseticAdapterAsset extends AsseticBaseAsset implements AssetInterface {
+abstract class AsseticAdapterAsset extends AsseticBaseAsset implements AsseticAssetInterface {
 
   /**
    * @throws \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
diff --git a/core/lib/Drupal/Core/Asset/Collection/AssetCollection.php b/core/lib/Drupal/Core/Asset/Collection/AssetCollection.php
index afaca2e..e795a95 100644
--- a/core/lib/Drupal/Core/Asset/Collection/AssetCollection.php
+++ b/core/lib/Drupal/Core/Asset/Collection/AssetCollection.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\Core\Asset\Collection;
+use Assetic\Asset\AssetInterface as AsseticAssetInterface;
 use Drupal\Core\Asset\Collection\AssetCollectionInterface;
 use Drupal\Core\Asset\AssetInterface;
 use Drupal\Core\Asset\Collection\Iterator\AssetSubtypeFilterIterator;
@@ -18,91 +19,16 @@
  *
  * TODO With PHP5.4, refactor out AssetCollectionBasicInterface into a trait.
  */
-class AssetCollection implements \IteratorAggregate, AssetCollectionInterface {
-
-  protected $assetStorage;
-
-  protected $assetIdMap = array();
+class AssetCollection extends BasicAssetCollection implements AssetCollectionInterface {
 
   protected $frozen = FALSE;
 
-  public function __construct() {
-    $this->assetStorage = new \SplObjectStorage();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function add(AssetInterface $asset) {
-    $this->attemptWrite();
-
-    if ($this->contains($asset) || $this->getById($asset->id())) {
-      return FALSE;
-    }
-
-    $this->assetStorage->attach($asset);
-    $this->assetIdMap[$asset->id()] = $asset;
-
-    return TRUE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function contains(AssetInterface $asset) {
-    // TODO decide whether to do this by id or object instance
-    return $this->assetStorage->contains($asset);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getById($id, $graceful = TRUE) {
-    if (isset($this->assetIdMap[$id])) {
-      return $this->assetIdMap[$id];
-    }
-    else if ($graceful) {
-      return FALSE;
-    }
-
-    throw new \OutOfBoundsException(sprintf('This collection does not contain an asset with id %s.', $id));
-  }
-
   /**
    * {@inheritdoc}
    */
-  public function remove($needle, $graceful = TRUE) {
-    // TODO fix horrible complexity of conditionals, exceptions, and returns.
+  public function add(AsseticAssetInterface $asset) {
     $this->attemptWrite();
-
-    // Validate and normalize type to AssetInterface
-    if (is_string($needle)) {
-      if (!$needle = $this->getById($needle, $graceful)) {
-        // Asset couldn't be found but we're in graceful mode - return FALSE.
-        return FALSE;
-      }
-    }
-    else if (!$needle instanceof AssetInterface) {
-      throw new \InvalidArgumentException('Invalid type provided to AssetCollection::remove(); must provide either a string asset id or AssetInterface instance.');
-    }
-
-    // Check for membership
-    if ($this->contains($needle)) {
-      unset($this->assetIdMap[$needle->id()], $this->assetStorage[$needle]);
-      return TRUE;
-    }
-    else if (!$graceful) {
-      throw new \OutOfBoundsException(sprintf('This collection does not contain an asset with id %s.', $needle->id()));
-    }
-
-    return FALSE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function all() {
-    return $this->assetIdMap;
+    return parent::add($asset);
   }
 
   /**
@@ -127,29 +53,31 @@ public function mergeCollection(AssetCollectionInterface $collection, $freeze =
   /**
    * {@inheritdoc}
    */
-  public function freeze() {
-    $this->frozen = TRUE;
+  public function remove($needle, $graceful = FALSE) {
+    $this->attemptWrite();
+    return parent::remove($needle, $graceful);
   }
 
   /**
    * {@inheritdoc}
    */
-  public function isFrozen() {
-    return $this->frozen;
+  public function replace($needle, AssetInterface $replacement, $graceful = FALSE) {
+    $this->attemptWrite();
+    return parent::replace($needle, $replacement, $graceful);
   }
 
   /**
    * {@inheritdoc}
    */
-  public function getIterator() {
-    return new \ArrayIterator($this->assetIdMap);
+  public function freeze() {
+    $this->frozen = TRUE;
   }
 
   /**
    * {@inheritdoc}
    */
-  public function isEmpty() {
-    return empty($this->assetIdMap);
+  public function isFrozen() {
+    return $this->frozen;
   }
 
   /**
@@ -157,7 +85,7 @@ public function isEmpty() {
    */
   public function getCss() {
     $collection = new self();
-    foreach (new AssetSubtypeFilterIterator($this->getIterator(), 'css') as $asset) {
+    foreach (new AssetSubtypeFilterIterator(new \ArrayIterator($this->all()), 'css') as $asset) {
       $collection->add($asset);
     }
 
@@ -169,7 +97,7 @@ public function getCss() {
    */
   public function getJs() {
     $collection = new self();
-    foreach (new AssetSubtypeFilterIterator($this->getIterator(), 'js') as $asset) {
+    foreach (new AssetSubtypeFilterIterator(new \ArrayIterator($this->all()), 'js') as $asset) {
       $collection->add($asset);
     }
 
diff --git a/core/lib/Drupal/Core/Asset/Collection/AssetCollectionBasicInterface.php b/core/lib/Drupal/Core/Asset/Collection/AssetCollectionBasicInterface.php
index b837b6e..e24ef3e 100644
--- a/core/lib/Drupal/Core/Asset/Collection/AssetCollectionBasicInterface.php
+++ b/core/lib/Drupal/Core/Asset/Collection/AssetCollectionBasicInterface.php
@@ -7,6 +7,9 @@
 
 namespace Drupal\Core\Asset\Collection;
 use Drupal\Core\Asset\AssetInterface;
+use Assetic\Asset\AssetInterface as AsseticAssetInterface;
+use Drupal\Core\Asset\Exception\AssetTypeMismatchException;
+use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
 
 /**
  * Describes an asset collection: a container for assets.
@@ -21,31 +24,36 @@
  * @see \Drupal\Core\Asset\Aggregate\AssetAggregateInterface
  * @see \Drupal\Core\Asset\Collection\AssetCollectionInterface
  */
-interface AssetCollectionBasicInterface extends \Traversable {
+interface AssetCollectionBasicInterface extends \Traversable, \Countable {
 
   /**
-   * Removes an asset from the aggregate.
+   * Adds an asset to this aggregate.
    *
-   * Wraps Assetic's AssetCollection::removeLeaf() to ease removal of keys.
-   *
-   * @param AssetInterface|string $needle
-   *   Either an AssetInterface instance, or the string id of an asset.
-   * @param bool $graceful
-   *   Whether failure should return FALSE or throw an exception.
+   * @param AsseticAssetInterface $asset
+   *   The asset to add. Note that, despite the type requirements, it must
+   *   conform to Drupal's AssetInterface.
    *
    * @return bool
+   *   TRUE if the asset was added successfully, FALSE if it was already present
+   *   in the aggregate.
    *
-   * @throws \OutOfBoundsException
+   * @throws UnsupportedAsseticBehaviorException
+   *   Thrown if a vanilla Assetic asset is provided.
+   *
+   * @throws AssetTypeMismatchException
+   *   Thrown if the provided asset is not the correct type for the aggregate
+   *   (e.g., CSS file in a JS aggregate).
    */
-  public function remove($needle, $graceful = FALSE);
+  public function add(AsseticAssetInterface $asset);
 
   /**
-   * Indicates whether this collection contains the provided asset.
-  *
+   * Indicates whether this collection contains the given asset.
+   *
    * @param AssetInterface $asset
-   *   Either an AssetInterface instance, or the string id of an asset.
+   *   The asset to check for membership in the collection.
    *
    * @return bool
+   *   TRUE if the asset is present in the collection, FALSE otherwise.
    */
   public function contains(AssetInterface $asset);
 
@@ -69,10 +77,95 @@ public function contains(AssetInterface $asset);
   public function getById($id, $graceful = TRUE);
 
   /**
-   * Indicates whether this collection contains any assets.
+   * Removes an asset from the collection.
+   *
+   * @param AssetInterface|string $needle
+   *   Either an AssetInterface instance, or the string id of an asset.
+   * @param bool $graceful
+   *   Whether failure should return FALSE or throw an exception.
    *
    * @return bool
-   *   TRUE if contained assets are present, FALSE otherwise.
+   *   TRUE on success, FALSE on failure to locate the given asset (or an
+   *   exception, depending on the value of $graceful).
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown if $needle could not be located and $graceful = FALSE.
+   */
+  public function remove($needle, $graceful = FALSE);
+
+  /**
+   * Replaces an existing asset in the aggregate with a new one.
+   *
+   * This preserves ordering of the assets within the collection: the new asset
+   * will occupy the same position as the old asset.
+   *
+   * @param AssetInterface|string $needle
+   *   Either an AssetInterface instance, or the string id of an asset.
+   * @param AssetInterface $replacement
+   *   The new asset to swap into place.
+   * @param bool $graceful
+   *   Whether failure should return FALSE or throw an exception.
+   *
+   * @return bool
+   *   TRUE on success, FALSE on failure to locate the given asset (or an
+   *   exception, depending on the value of $graceful).
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown if $needle could not be located and $graceful = FALSE.
+   */
+  public function replace($needle, AssetInterface $replacement, $graceful = FALSE);
+
+  /**
+   * Indicates whether the collection contains any assets.
+   *
+   * Note that this will only return TRUE if leaf assets are present - that is,
+   * assets that do NOT implement AssetCollectionBasicInterface.
+   *
+   * @return bool
+   *   TRUE if the collection is devoid of any leaf assets, FALSE otherwise.
    */
   public function isEmpty();
-}
\ No newline at end of file
+
+  /**
+   * Returns all top-level child assets as an array.
+   *
+   * To retrieve assets regardless of nesting level, see the iterators:
+   *
+   * @see AssetCollectionBasicInterface::each()
+   * @see AssetCollectionBasicInterface::eachLeaf()
+   *
+   * @return AssetInterface[]
+   */
+  public function all();
+
+  /**
+   * Returns the total number of leaf assets in this collection.
+   *
+   * Non-leaf assets - objects implementing AssetCollectionBasicInterface - are
+   * not included in the count.
+   *
+   * @return int
+   */
+  public function count();
+
+  /**
+   * Retrieves a traversable that will return all contained assets.
+   *
+   * 'All' assets includes both AssetCollectionBasicInterface objects and plain
+   * AssetInterface objects.
+   *
+   * @return \Traversable
+   */
+  public function each();
+
+  /**
+   * Retrieves a traversable that returns only contained leaf assets.
+   *
+   * Leaf assets are objects that only implement AssetInterface, not
+   * AssetCollectionBasicInterface.
+   *
+   * @return \Traversable
+   */
+  public function eachLeaf();
+}
+
diff --git a/core/lib/Drupal/Core/Asset/Collection/AssetCollectionInterface.php b/core/lib/Drupal/Core/Asset/Collection/AssetCollectionInterface.php
index 8fb6f1a..6b7a8ce 100644
--- a/core/lib/Drupal/Core/Asset/Collection/AssetCollectionInterface.php
+++ b/core/lib/Drupal/Core/Asset/Collection/AssetCollectionInterface.php
@@ -12,32 +12,13 @@
 /**
  * Describes an asset collection.
  *
- * TODO we need a few more methods here to deal with asset type disambiguation and library resolution
+ * TODO we need a few more methods here to deal with library resolution
  *
  * @see \Drupal\Core\Asset\Collection\AssetCollectionBasicInterface
  */
 interface AssetCollectionInterface extends AssetCollectionBasicInterface {
 
   /**
-   * Returns all assets contained in this collection.
-   *
-   * @return AssetInterface[]
-   */
-  public function all();
-
-  /**
-   * Adds an asset to the collection.
-   *
-   * @param \Drupal\Core\Asset\AssetInterface $asset
-   *   The asset to add.
-   *
-   * @return bool
-   *   TRUE if the asset was added successfully, FALSE if it was already present
-   *   in the collection.
-   */
-  public function add(AssetInterface $asset);
-
-  /**
    * Merges another asset collection into this one.
    *
    * If an asset is present in both collections, as identified by
diff --git a/core/lib/Drupal/Core/Asset/Collection/BasicAssetCollection.php b/core/lib/Drupal/Core/Asset/Collection/BasicAssetCollection.php
new file mode 100644
index 0000000..034ea07
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Collection/BasicAssetCollection.php
@@ -0,0 +1,321 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\Collection\BasicAssetCollection.
+ */
+
+namespace Drupal\Core\Asset\Collection;
+
+use Drupal\Core\Asset\Metadata\AssetMetadataInterface;
+use Drupal\Core\Asset\Collection\Iterator\RecursiveBasicCollectionIterator;
+use Drupal\Core\Asset\AssetInterface;
+use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
+use Assetic\Asset\AssetInterface as AsseticAssetInterface;
+
+/**
+ * Base class implementing AssetCollectionBasicInterface.
+ *
+ * This class provides the essentials of the asset collection implementation,
+ * common to all of the collection flavors.
+ *
+ * TODO With PHP5.4, refactor this entire thing into a trait.
+ */
+abstract class BasicAssetCollection implements \IteratorAggregate, AssetCollectionBasicInterface {
+
+  /**
+   * A map of all assets, keyed by asset id.
+   *
+   * This map is also the canonical source for ordering information.
+   *
+   * @var array
+   */
+  protected $assetIdMap = array();
+
+  /**
+   * Container for all assets held within this object.
+   *
+   * @var \SplObjectStorage
+   */
+  protected $assetStorage;
+
+  /**
+   * Container for all nested asset collections held within in this object.
+   *
+   * @var \SplObjectStorage
+   */
+  protected $nestedStorage;
+
+  /**
+   * @param AssetInterface[] $assets
+   *   (optional) An array of assets to immediately add to this collection.
+   */
+  public function __construct($assets = array()) {
+    $this->assetStorage = new \SplObjectStorage();
+    $this->nestedStorage = new \SplObjectStorage();
+
+    foreach ($assets as $asset) {
+      $this->add($asset);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function add(AsseticAssetInterface $asset) {
+    if (!$asset instanceof AssetInterface) {
+      throw new UnsupportedAsseticBehaviorException('Vanilla Assetic asset provided; Drupal collections require Drupal-flavored assets.');
+    }
+    $this->ensureCorrectType($asset);
+
+    if ($this->contains($asset) || $this->getById($asset->id())) {
+      return FALSE;
+    }
+
+    $this->assetStorage->attach($asset);
+    $this->assetIdMap[$asset->id()] = $asset;
+
+    if ($asset instanceof AssetCollectionBasicInterface) {
+      $this->nestedStorage->attach($asset);
+    }
+
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function contains(AssetInterface $asset) {
+    if ($this->assetStorage->contains($asset)) {
+      return TRUE;
+    }
+
+    foreach ($this->nestedStorage as $aggregate) {
+      if ($aggregate->contains($asset)) {
+        return TRUE;
+      }
+    }
+
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getById($id, $graceful = TRUE) {
+    if (isset($this->assetIdMap[$id])) {
+      return $this->assetIdMap[$id];
+    }
+    else {
+      // Recursively search for the id
+      foreach ($this->nestedStorage as $aggregate) {
+        if ($found = $aggregate->getById($id)) {
+          return $found;
+        }
+      }
+    }
+
+    if ($graceful) {
+      return FALSE;
+    }
+
+    throw new \OutOfBoundsException(sprintf('This collection does not contain an asset with id %s.', $id));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function remove($needle, $graceful = FALSE) {
+    if (is_string($needle)) {
+      if (!$needle = $this->getById($needle, $graceful)) {
+        return FALSE;
+      }
+    }
+    else if (!$needle instanceof AssetInterface) {
+      throw new \InvalidArgumentException('Invalid type provided to AssetCollectionBasicInterface::replace(); must provide either a string asset id or AssetInterface instance.');
+    }
+
+    return $this->doRemove($needle, $graceful);
+  }
+
+  /**
+   * Performs the actual work of removing an asset from the collection.
+   *
+   * @param AssetInterface|string $needle
+   *   Either an AssetInterface instance, or the string id of an asset.
+   * @param bool $graceful
+   *   Whether failure should return FALSE or throw an exception.
+   *
+   * @return bool
+   *   TRUE on success, FALSE on failure to locate the given asset (or an
+   *   exception, depending on the value of $graceful).
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown if $needle could not be located and $graceful = FALSE.
+   */
+  protected function doRemove(AssetInterface $needle, $graceful) {
+    foreach ($this->assetIdMap as $id => $asset) {
+      if ($asset === $needle) {
+        unset($this->assetStorage[$asset], $this->assetIdMap[$id], $this->nestedStorage[$asset]);
+
+        return TRUE;
+      }
+
+      if ($asset instanceof AssetCollectionBasicInterface && $asset->doRemove($needle, TRUE)) {
+        return TRUE;
+      }
+    }
+
+    if ($graceful) {
+      return FALSE;
+    }
+
+    throw new \OutOfBoundsException('Provided asset was not found in the collection.');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function replace($needle, AssetInterface $replacement, $graceful = FALSE) {
+    if (is_string($needle)) {
+      if (!$needle = $this->getById($needle, $graceful)) {
+        return FALSE;
+      }
+    }
+    else if (!$needle instanceof AssetInterface) {
+      throw new \InvalidArgumentException('Invalid type provided to AssetCollectionBasicInterface::replace(); must provide either a string asset id or AssetInterface instance.');
+    }
+
+    $this->ensureCorrectType($replacement);
+    if ($this->contains($replacement)) {
+      throw new \LogicException('Asset to be swapped in is already present in the collection.');
+    }
+
+    return $this->doReplace($needle, $replacement, $graceful);
+  }
+
+  /**
+   * Performs the actual work of replacing one asset with another.
+   *
+   * @param AssetInterface $needle
+   *   The AssetInterface instance to swap out.
+   * @param AssetInterface $replacement
+   *   The new asset to swap in.
+   * @param bool $graceful
+   *   Whether failure should return FALSE or throw an exception.
+   *
+   * @return bool
+   *   TRUE on success, FALSE on failure to locate the given asset (or an
+   *   exception, depending on the value of $graceful).
+   *
+   * @throws \OutOfBoundsException
+   */
+  protected function doReplace(AssetInterface $needle, AssetInterface $replacement, $graceful) {
+    $i = 0;
+    foreach ($this->assetIdMap as $id => $asset) {
+      if ($asset === $needle) {
+        unset($this->assetStorage[$asset], $this->nestedStorage[$asset]);
+
+        array_splice($this->assetIdMap, $i, 1, array($replacement->id() => $replacement));
+        $this->assetStorage->attach($replacement);
+        if ($replacement instanceof AssetCollectionBasicInterface) {
+          $this->nestedStorage->attach($replacement);
+        }
+
+        return TRUE;
+      }
+
+      if ($asset instanceof AssetCollectionBasicInterface && $asset->doReplace($needle, $replacement, TRUE)) {
+        return TRUE;
+      }
+      $i++;
+    }
+
+    if ($graceful) {
+      return FALSE;
+    }
+
+    throw new \OutOfBoundsException('Provided asset was not found in the collection.');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function all() {
+    return $this->assetIdMap;
+  }
+
+  /**
+   * {@inheritdoc}
+   * TODO Assetic uses their iterator to clone, then populate values and return here; is that a good model for us?
+   */
+  public function getIterator() {
+    return new \RecursiveIteratorIterator(new RecursiveBasicCollectionIterator($this), \RecursiveIteratorIterator::SELF_FIRST);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function each() {
+    return $this->getIterator();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function eachLeaf() {
+    return new \RecursiveIteratorIterator(new RecursiveBasicCollectionIterator($this));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    $maincount = $this->assetStorage->count();
+    if ($maincount === 0) {
+      return TRUE;
+    }
+
+    $i = 0;
+    foreach ($this->nestedStorage as $aggregate) {
+      if (!$aggregate->isEmpty()) {
+        return FALSE;
+      }
+      $i++;
+    }
+
+    return $i === $maincount;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function count() {
+    if ($this->nestedStorage->count() === 0) {
+      return $this->assetStorage->count();
+    }
+
+    $c = $i = 0;
+    foreach ($this->nestedStorage as $collection) {
+      $c += $collection->count();
+      $i++;
+    }
+
+    return $this->assetStorage->count() - $i + $c;
+  }
+
+  /**
+   * Ensures that the asset is the correct type for this collection.
+   *
+   * "Type" here refers to 'css' vs. 'js'.
+   *
+   * BasicAssetCollection's implementation has no body because it has no type
+   * restrictions; only aggregates do.
+   *
+   * @param AssetInterface $asset
+   *
+   * @throws \Drupal\Core\Asset\Exception\AssetTypeMismatchException
+   */
+  protected function ensureCorrectType(AssetInterface $asset) {}
+}
+
diff --git a/core/lib/Drupal/Core/Asset/Collection/Iterator/RecursiveBasicCollectionIterator.php b/core/lib/Drupal/Core/Asset/Collection/Iterator/RecursiveBasicCollectionIterator.php
new file mode 100644
index 0000000..2557a03
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Collection/Iterator/RecursiveBasicCollectionIterator.php
@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\Aggregate\Iterator\RecursiveBasicCollectionIterator.
+ */
+
+namespace Drupal\Core\Asset\Collection\Iterator;
+
+use Drupal\Core\Asset\Collection\AssetCollectionBasicInterface;
+
+/**
+ * Iterates over an AssetCollectionBasicInterface, treating only assets
+ * that themselves implement AssetCollectionBasicInterface as having children.
+ */
+class RecursiveBasicCollectionIterator extends \RecursiveArrayIterator {
+  public function __construct(AssetCollectionBasicInterface $collection) {
+    parent::__construct($collection->all());
+  }
+
+  public function hasChildren() {
+    return $this->current() instanceof AssetCollectionBasicInterface;
+  }
+}
\ No newline at end of file
diff --git a/core/tests/Drupal/Tests/Core/Asset/Aggregate/AssetAggregateTest.php b/core/tests/Drupal/Tests/Core/Asset/Aggregate/AssetAggregateTest.php
new file mode 100644
index 0000000..e6a90a7
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/Aggregate/AssetAggregateTest.php
@@ -0,0 +1,324 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Asset\Aggregate\AssetAggregateTest.
+ */
+
+namespace Drupal\Tests\Core\Asset\Aggregate;
+
+use Drupal\Core\Asset\Collection\AssetCollectionBasicInterface;
+use Drupal\Core\Asset\Exception\AssetTypeMismatchException;
+use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
+use Drupal\Tests\Core\Asset\Collection\BasicAssetCollectionTest;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Asset\Aggregate\AssetAggregate
+ * @group Asset
+ */
+class AssetAggregateTest extends BasicAssetCollectionTest {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Asset aggregate tests',
+      'description' => 'Unit tests on AssetAggregate',
+      'group' => 'Asset',
+    );
+  }
+
+  /**
+   * Generates a AssetAggregate mock with three leaf assets.
+   */
+  public function getThreeLeafAggregate() {
+    $aggregate = $this->getAggregate();
+    $nested_aggregate = $this->getAggregate();
+
+    foreach (array('foo', 'bar', 'baz') as $var) {
+      $$var = $this->createStubFileAsset('css', $var);
+    }
+
+    $nested_aggregate->add($foo);
+    $nested_aggregate->add($bar);
+    $aggregate->add($nested_aggregate);
+    $aggregate->add($baz);
+
+    return array($aggregate, $foo, $bar, $baz, $nested_aggregate);
+  }
+
+  /**
+   * Returns an AssetAggregate, the base collection type for this unit test.
+   *
+   * @return AssetCollectionBasicInterface
+   */
+  public function getCollection() {
+    return $this->getAggregate();
+  }
+
+  public function testGetAssetType() {
+    $mockmeta = $this->getMock('\\Drupal\\Core\\Asset\\Metadata\\AssetMetadataBag', array(), array(), '', FALSE);
+    $mockmeta->expects($this->once())
+      ->method('getType')
+      ->will($this->returnValue('unicorns'));
+    $aggregate = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\AssetAggregate', array($mockmeta));
+
+    $this->assertEquals('unicorns', $aggregate->getAssetType());
+  }
+
+  public function testGetMetadata() {
+    $mockmeta = $this->createStubAssetMetadata();
+    $aggregate = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\AssetAggregate', array($mockmeta));
+
+    $this->assertSame($mockmeta, $aggregate->getMetadata());
+  }
+
+  /**
+   * This uses PHPUnit's reflection-based assertions rather than assertContains
+   * so that this test can honestly sit at the root of the test method
+   * dependency tree.
+   *
+   * @covers ::add
+   */
+  public function testAdd() {
+    $aggregate = $this->getAggregate();
+    $asset = $this->createStubFileAsset();
+    $this->assertTrue($aggregate->add($asset));
+
+    $this->assertAttributeContains($asset, 'assetStorage', $aggregate);
+    $this->assertAttributeContains($asset, 'assetIdMap', $aggregate);
+
+    // Nesting: add an aggregate to the first aggregate.
+    $nested_aggregate = $this->getAggregate();
+    $aggregate->add($nested_aggregate);
+
+    $this->assertAttributeContains($nested_aggregate, 'assetStorage', $aggregate);
+    $this->assertAttributeContains($nested_aggregate, 'assetIdMap', $aggregate);
+    $this->assertAttributeContains($nested_aggregate, 'nestedStorage', $aggregate);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::ensureCorrectType
+   * @expectedException \Drupal\Core\Asset\Exception\AssetTypeMismatchException
+   */
+  public function testAddEnsureCorrectType() {
+    $aggregate = $this->getAggregate();
+    $aggregate->add($this->createStubFileAsset('js'));
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::each
+   * @covers ::getIterator
+   * @covers \Drupal\Core\Asset\Collection\Iterator\RecursiveBasicCollectionIterator
+   */
+  public function testEach() {
+    list($aggregate, $foo, $bar, $baz, $nested_aggregate) = $this->getThreeLeafAggregate();
+
+    $contained = array();
+    foreach ($aggregate->each() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($nested_aggregate, $foo, $bar, $baz), $contained);
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testEach
+   * @covers ::__construct
+   */
+  public function testCreateWithAssets() {
+    $asset1 = $this->createStubFileAsset();
+    $asset2 = $this->createStubFileAsset();
+    $meta = $this->createStubAssetMetadata();
+    $collection = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\AssetAggregate', array($meta, array($asset1, $asset2)));
+
+    $this->assertContains($asset1, $collection);
+    $this->assertContains($asset2, $collection);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::id
+   * @covers ::calculateId
+   */
+  public function testId() {
+    // Simple case - test with one contained asset first.
+    $aggregate = $this->getAggregate();
+    $asset1 = $this->createStubFileAsset();
+    $aggregate->add($asset1);
+
+    $this->assertEquals(hash('sha256', $asset1->id()), $aggregate->id());
+
+    // Now use two contained assets, one nested in another aggregate.
+    $aggregate = $this->getAggregate();
+    $aggregate->add($asset1);
+
+    $aggregate2 = $this->getAggregate();
+    $asset2 = $this->createStubFileAsset();
+    $aggregate2->add($asset2);
+
+    $aggregate->add($aggregate2);
+
+    // The aggregate only uses leaf, non-aggregate assets to determine its id.
+    $this->assertEquals(hash('sha256', $asset1->id() . $asset2->id()), $aggregate->id());
+  }
+
+  public function testIsPreprocessable() {
+    $this->assertTrue($this->getAggregate()->isPreprocessable());
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::removeLeaf
+   * @expectedException \OutOfBoundsException
+   */
+  public function testRemoveNonexistentNeedle() {
+    list($aggregate) = $this->getThreeLeafAggregate();
+    // Nonexistent leaf removal returns FALSE in graceful mode
+    $this->assertFalse($aggregate->removeLeaf($this->createStubFileAsset(), TRUE));
+
+    // In non-graceful mode, an exception is thrown.
+    $aggregate->removeLeaf($this->createStubFileAsset());
+  }
+
+  /**
+   * @covers ::removeLeaf
+   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public function testRemoveLeafVanillaAsseticAsset() {
+    $aggregate = $this->getAggregate();
+    $vanilla = $this->getMock('\\Assetic\\Asset\\BaseAsset', array(), array(), '', FALSE);
+    $aggregate->removeLeaf($vanilla);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::ensureCorrectType
+   * @expectedException \Drupal\Core\Asset\Exception\AssetTypeMismatchException
+   */
+  public function testReplaceLeafEnsureCorrectType() {
+    $aggregate = $this->getAggregate();
+    $asset1 = $this->createStubFileAsset();
+    $aggregate->add($asset1);
+
+    $asset2 = $this->createStubFileAsset('js');
+    $aggregate->replaceLeaf($asset1, $asset2);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::ensureCorrectType
+   * @expectedException \Drupal\Core\Asset\Exception\AssetTypeMismatchException
+   */
+  public function testReplaceEnsureCorrectType() {
+    $aggregate = $this->getAggregate();
+    $asset1 = $this->createStubFileAsset();
+    $aggregate->add($asset1);
+
+    $asset2 = $this->createStubFileAsset('js');
+    $aggregate->replace($asset1, $asset2);
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::replaceLeaf
+   * @expectedException \OutOfBoundsException
+   */
+  public function testReplaceLeafNonexistentNeedle() {
+    list($aggregate) = $this->getThreeLeafAggregate();
+    // Nonexistent leaf replacement returns FALSE in graceful mode
+    $qux = $this->createStubFileAsset();
+    $this->assertFalse($aggregate->replaceLeaf($this->createStubFileAsset(), $qux, TRUE));
+    $this->assertNotContains($qux, $aggregate);
+
+    // In non-graceful mode, an exception is thrown.
+    $aggregate->replaceLeaf($this->createStubFileAsset(), $qux);
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::replaceLeaf
+   * @expectedException \LogicException
+   */
+  public function testReplaceLeafWithAlreadyPresentAsset() {
+    list($aggregate, $foo) = $this->getThreeLeafAggregate();
+    $aggregate->replaceLeaf($this->createStubFileAsset(), $foo);
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testReplaceLeafWithAlreadyPresentAsset
+   * @covers ::replace
+   * @expectedException \LogicException
+   *
+   * This fails on the same check that testReplaceLeafWithAlreadyPresentAsset,
+   * but it is demonstrated as its own test for clarity.
+   */
+  public function testReplaceLeafWithSelf() {
+    list($aggregate, $foo) = $this->getThreeLeafAggregate();
+    $aggregate->replaceLeaf($foo, $foo);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::replaceLeaf
+   */
+  public function testReplaceLeafVanillaAsseticAsset() {
+    $aggregate = $this->getAggregate();
+    $vanilla = $this->getMock('\\Assetic\\Asset\\BaseAsset', array(), array(), '', FALSE);
+    $drupally = $this->createStubFileAsset();
+
+    try {
+      $aggregate->replaceLeaf($vanilla, $drupally);
+      $this->fail('AssetAggregate::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
+    } catch (UnsupportedAsseticBehaviorException $e) {}
+
+    try {
+      $aggregate->replaceLeaf($vanilla, $vanilla);
+      $this->fail('AssetAggregate::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
+    } catch (UnsupportedAsseticBehaviorException $e) {}
+
+    try {
+      $aggregate->replaceLeaf($drupally, $vanilla);
+      $this->fail('AssetAggregate::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
+    } catch (UnsupportedAsseticBehaviorException $e) {}
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::load
+   */
+  public function testLoad() {
+    $this->fail();
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::dump
+   */
+  public function testDump() {
+    $this->fail();
+  }
+
+  /**
+   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public function testGetVars() {
+    $this->getAggregate()->getVars();
+  }
+
+  /**
+   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public function testSetValues() {
+    $this->getAggregate()->setValues(array());
+  }
+
+  /**
+   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   */
+  public function testGetValues() {
+    $this->getAggregate()->getValues();
+  }
+}
+
diff --git a/core/tests/Drupal/Tests/Core/Asset/Aggregate/BaseAggregateAssetTest.php b/core/tests/Drupal/Tests/Core/Asset/Aggregate/BaseAggregateAssetTest.php
deleted file mode 100644
index ff8ba4a..0000000
--- a/core/tests/Drupal/Tests/Core/Asset/Aggregate/BaseAggregateAssetTest.php
+++ /dev/null
@@ -1,377 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Tests\Core\Asset\Aggregate\BaseAggregateAssetTest.
- */
-
-namespace Drupal\Tests\Core\Asset\Aggregate;
-
-use Drupal\Core\Asset\Aggregate\BaseAggregateAsset;
-use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
-use Drupal\Tests\Core\Asset\AssetUnitTest;
-
-/**
- * @coversDefaultClass \Drupal\Core\Asset\Aggregate\BaseAggregateAsset
- * @group Asset
- */
-class BaseAggregateAssetTest extends AssetUnitTest {
-
-  protected $aggregate;
-
-  public static function getInfo() {
-    return array(
-      'name' => 'Asset aggregate tests',
-      'description' => 'Unit tests on BaseAggregateAsset',
-      'group' => 'Asset',
-    );
-  }
-
-  /**
-   * Generates a simple BaseAggregateAsset mock.
-   *
-   * @param array $defaults
-   *   Defaults to inject into the aggregate's metadata bag.
-   *
-   * @return BaseAggregateAsset
-   */
-  public function getAggregate($defaults = array()) {
-    $mockmeta = $this->createStubAssetMetadata();
-    return $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\BaseAggregateAsset', array($mockmeta));
-  }
-
-  /**
-   * Generates a BaseAggregateAsset mock with three leaf assets.
-   */
-  public function getThreeLeafAggregate() {
-    $aggregate = $this->getAggregate();
-    $nested_aggregate = $this->getAggregate();
-
-    foreach (array('foo', 'bar', 'baz') as $var) {
-      $$var = $this->getMock('Drupal\\Core\\Asset\\FileAsset', array(), array(), '', FALSE);
-      $$var->expects($this->any())
-        ->method('id')
-        ->will($this->returnValue($var));
-    }
-
-    $aggregate->add($foo);
-    $nested_aggregate->add($bar);
-    $nested_aggregate->add($baz);
-    $aggregate->add($nested_aggregate);
-
-    return array($aggregate, $foo, $bar, $baz);
-  }
-
-  public function testGetAssetType() {
-    $mockmeta = $this->getMock('\\Drupal\\Core\\Asset\\Metadata\\AssetMetadataBag', array(), array(), '', FALSE);
-    $mockmeta->expects($this->once())
-      ->method('getType')
-      ->will($this->returnValue('unicorns'));
-    $aggregate = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\BaseAggregateAsset', array($mockmeta));
-
-    $this->assertEquals('unicorns', $aggregate->getAssetType());
-  }
-
-  public function testGetMetadata() {
-    $mockmeta = $this->createStubAssetMetadata();
-    $aggregate = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\BaseAggregateAsset', array($mockmeta));
-
-    $this->assertSame($mockmeta, $aggregate->getMetadata());
-  }
-
-  /**
-   * @covers ::add
-   */
-  public function testAdd() {
-    $aggregate = $this->getAggregate();
-    $asset = $this->createMockFileAsset('css');
-    $this->assertTrue($aggregate->add($asset));
-
-    $this->assertContains($asset, $aggregate);
-
-    // Nesting: add an aggregate to the first aggregate.
-    $nested_aggregate = $this->getAggregate();
-    $nested_asset = $this->createMockFileAsset('css');
-
-    $nested_aggregate->add($nested_asset);
-    $aggregate->add($nested_asset);
-
-    $this->assertContains($nested_asset, $aggregate);
-  }
-
-  /**
-   * Tests that adding the same asset twice is disallowed.
-   *
-   * @depends testAdd
-   * @covers ::add
-   */
-  public function testDoubleAdd() {
-    $aggregate = $this->getAggregate();
-    $asset = $this->createMockFileAsset('css');
-    $this->assertTrue($aggregate->add($asset));
-
-    // Test by object identity
-    $this->assertFalse($aggregate->add($asset));
-    // Test by id
-    $asset2 = $this->getMock('Drupal\\Core\\Asset\\FileAsset', array(), array(), '', FALSE);
-    $asset2->expects($this->once())
-      ->method('id')
-      ->will($this->returnValue($asset->id()));
-
-    $this->assertFalse($aggregate->add($asset2));
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::contains
-   */
-  public function testContains() {
-    $aggregate = $this->getAggregate();
-    $asset = $this->createMockFileAsset('css');
-    $aggregate->add($asset);
-
-    $this->assertTrue($aggregate->contains($asset));
-
-    // Nesting: add an aggregate to the first aggregate.
-    $nested_aggregate = $this->getAggregate();
-    $nested_asset = $this->createMockFileAsset('css');
-
-    $nested_aggregate->add($nested_asset);
-    $aggregate->add($nested_aggregate);
-
-    $this->assertTrue($aggregate->contains($nested_asset));
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::id
-   * @covers ::calculateId
-   */
-  public function testId() {
-    // Simple case - test with one contained asset first.
-    $aggregate = $this->getAggregate();
-    $asset1 = $this->createMockFileAsset('css');
-    $aggregate->add($asset1);
-
-    $this->assertEquals(hash('sha256', $asset1->id()), $aggregate->id());
-
-    // Now use two contained assets, one nested in another aggregate.
-    $aggregate = $this->getAggregate();
-    $aggregate->add($asset1);
-
-    $aggregate2 = $this->getAggregate();
-    $asset2 = $this->createMockFileAsset('css');
-    $aggregate2->add($asset2);
-
-    $aggregate->add($aggregate2);
-
-    // The aggregate only uses leaf, non-aggregate assets to determine its id.
-    $this->assertEquals(hash('sha256', $asset1->id() . $asset2->id()), $aggregate->id());
-  }
-
-  /**
-   * @covers ::getById
-   * @expectedException \OutOfBoundsException
-   */
-  public function testGetById() {
-    $aggregate = $this->getAggregate();
-
-    $asset = $this->createMockFileAsset('css');
-    $aggregate->add($asset);
-    $this->assertSame($asset, $aggregate->getById($asset->id()));
-
-    // Nonexistent asset
-    $this->assertFalse($aggregate->getById('bar'));
-
-    // Nonexistent asset, non-graceful
-    $aggregate->getById('bar', FALSE);
-  }
-
-  public function testIsPreprocessable() {
-    $this->assertTrue($this->getAggregate()->isPreprocessable());
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::all
-   */
-  public function testAll() {
-    $aggregate = $this->getAggregate();
-
-    $asset1 = $this->createMockFileAsset('css');
-    $asset2 = $this->createMockFileAsset('css');
-    $aggregate->add($asset1);
-    $aggregate->add($asset2);
-
-    $output = array(
-      $asset1->id() => $asset1,
-      $asset2->id() => $asset2,
-    );
-
-    $this->assertEquals($output, $aggregate->all());
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::replace
-   * @expectedException \OutOfBoundsException
-   */
-  public function testIsEmpty() {
-    $aggregate = $this->getAggregate();
-    $this->assertTrue($aggregate->isEmpty());
-
-    // Aggregates containing only empty aggregates are considered empty.
-    $aggregate->add($this->getAggregate());
-    $this->assertTrue($aggregate->isEmpty());
-  }
-
-  /**
-   * remove() and removeLeaf() are conjoined; test them both here.
-   *
-   * @depends testAdd
-   * @covers ::remove
-   * @covers ::removeLeaf
-   */
-  public function testRemove() {
-    list($aggregate, $foo, $bar, $baz) = $this->getThreeLeafAggregate();
-    $this->assertTrue($aggregate->remove('foo'));
-
-    $this->assertNotContains($foo, $aggregate);
-    $this->assertContains($bar, $aggregate);
-    $this->assertContains($baz, $aggregate);
-
-    $this->assertTrue($aggregate->remove($bar));
-
-    $this->assertNotContains($bar, $aggregate);
-    $this->assertContains($baz, $aggregate);
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::removeLeaf
-   * @expectedException \OutOfBoundsException
-   */
-  public function testRemoveNonexistentNeedle() {
-    list($aggregate,,,) = $this->getThreeLeafAggregate();
-    // Nonexistent leaf removal returns FALSE in graceful mode
-    $this->assertFalse($aggregate->removeLeaf($this->createMockFileAsset('css')));
-
-    // In non-graceful mode, an exception is thrown.
-    $aggregate->removeLeaf($this->createMockFileAsset('css'), FALSE);
-  }
-
-  /**
-   * @covers ::removeLeaf
-   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
-   */
-  public function testRemoveLeafVanillaAsseticAsset() {
-    $aggregate = $this->getAggregate();
-    $vanilla = $this->getMock('\\Assetic\\Asset\\BaseAsset', array(), array(), '', FALSE);
-    $aggregate->removeLeaf($vanilla);
-  }
-
-  /**
-   * replace() and replaceLeaf() are conjoined; test them both here.
-   *
-   * @depends testAdd
-   * @covers ::replace
-   * @covers ::replaceLeaf
-   */
-  public function testReplace() {
-    list($aggregate, $foo, $bar, $baz) = $this->getThreeLeafAggregate();
-    $qux = $this->getMock('Drupal\\Core\\Asset\\FileAsset', array(), array(), '', FALSE);
-    $qux->expects($this->any())
-      ->method('id')
-      ->will($this->returnValue('qux'));
-
-    $this->assertTrue($aggregate->replace('foo', $qux));
-
-    $this->assertContains($qux, $aggregate);
-    $this->assertNotContains($foo, $aggregate);
-
-    $contained = array();
-    foreach ($aggregate as $leaf) {
-      $contained[] = $leaf;
-    }
-    $this->assertEquals(array($qux, $bar, $baz), $contained);
-
-    $this->assertTrue($aggregate->replace($bar, $foo));
-
-    $this->assertContains($foo, $aggregate);
-    $this->assertNotContains($bar, $aggregate);
-
-    $contained = array();
-    foreach ($aggregate as $leaf) {
-      $contained[] = $leaf;
-    }
-    $this->assertEquals(array($qux, $foo, $baz), $contained);
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::replaceLeaf
-   * @expectedException \OutOfBoundsException
-   */
-  public function testReplaceLeafNonexistentNeedle() {
-    list($aggregate,,,) = $this->getThreeLeafAggregate();
-    // Nonexistent leaf replacement returns FALSE in graceful mode
-    $qux = $this->createMockFileAsset('css');
-    $this->assertFalse($aggregate->replaceLeaf($this->createMockFileAsset('css'), $qux));
-    $this->assertNotContains($qux, $aggregate);
-
-    // In non-graceful mode, an exception is thrown.
-    $aggregate->replaceLeaf($this->createMockFileAsset('css'), $qux, FALSE);
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::replaceLeaf
-   * @expectedException \LogicException
-   */
-  public function testReplaceLeafWithAlreadyPresentAsset() {
-    list($aggregate, $foo, $bar, $baz) = $this->getThreeLeafAggregate();
-    $aggregate->replaceLeaf($foo, $foo);
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::replaceLeaf
-   */
-  public function testReplaceLeafVanillaAsseticAsset() {
-    $aggregate = $this->getAggregate();
-    $vanilla = $this->getMock('\\Assetic\\Asset\\BaseAsset', array(), array(), '', FALSE);
-    $drupally = $this->createMockFileAsset('css');
-
-    try {
-      $aggregate->replaceLeaf($vanilla, $drupally);
-      $this->fail('BaseAggregateAsset::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
-    } catch (UnsupportedAsseticBehaviorException $e) {}
-
-    try {
-      $aggregate->replaceLeaf($vanilla, $vanilla);
-      $this->fail('BaseAggregateAsset::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
-    } catch (UnsupportedAsseticBehaviorException $e) {}
-
-    try {
-      $aggregate->replaceLeaf($drupally, $vanilla);
-      $this->fail('BaseAggregateAsset::removeLeaf() did not throw an UnsupportedAsseticBehaviorException when provided a vanilla asset leaf.');
-    } catch (UnsupportedAsseticBehaviorException $e) {}
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::load
-   */
-  public function testLoad() {
-    $this->fail();
-  }
-
-  /**
-   * @depends testAdd
-   * @covers ::dump
-   */
-  public function testDump() {
-    $this->fail();
-  }
-}
-
diff --git a/core/tests/Drupal/Tests/Core/Asset/AssetUnitTest.php b/core/tests/Drupal/Tests/Core/Asset/AssetUnitTest.php
index f2bc236..c34b382 100644
--- a/core/tests/Drupal/Tests/Core/Asset/AssetUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/AssetUnitTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\Core\Asset;
 
+use Drupal\Core\Asset\Aggregate\AssetAggregate;
 use Drupal\Core\Asset\FileAsset;
 use Drupal\Core\Asset\Metadata\AssetMetadataBag;
 use Drupal\Tests\UnitTestCase;
@@ -23,11 +24,15 @@
    * id(), with a randomly generated name.
    *
    * @param string $type
-   *   'css' or 'js'.
+   *   'css' or 'js'. Defaults to 'css' if not given.
+   *
+   * @param string $id
+   *   A string id for the asset, to return from AssetInterface::id(). Defaults
+   *   to a random string if not given.
    *
    * @return FileAsset
    */
-  public function createMockFileAsset($type) {
+  public function createStubFileAsset($type = 'css', $id = '') {
     $asset = $this->getMock('Drupal\\Core\\Asset\\FileAsset', array(), array(), '', FALSE);
     $asset->expects($this->any())
       ->method('getAssetType')
@@ -35,7 +40,7 @@ public function createMockFileAsset($type) {
 
     $asset->expects($this->any())
       ->method('id')
-      ->will($this->returnValue($this->randomName()));
+      ->will($this->returnValue($id ?: $this->randomName()));
 
     return $asset;
   }
@@ -49,9 +54,29 @@ public function createMockFileAsset($type) {
    * @return AssetMetadataBag
    */
   public function createStubAssetMetadata($type = 'css', $values = array()) {
-    return $this->getMockBuilder('Drupal\\Core\\Asset\\Metadata\\AssetMetadataBag')
+    $stub = $this->getMockBuilder('Drupal\\Core\\Asset\\Metadata\\AssetMetadataBag')
       ->setConstructorArgs(array($type, $values))
-      ->setMethods(array()) // mock nothing
       ->getMock();
+
+    $stub->expects($this->any())
+      ->method('getType')
+      ->will($this->returnValue($type));
+
+    return $stub;
+  }
+
+
+  /**
+   * Generates a simple AssetAggregate mock.
+   *
+   * @param array $defaults
+   *   Defaults to inject into the aggregate's metadata bag.
+   *
+   * @return AssetAggregate
+   */
+  public function getAggregate($defaults = array()) {
+    $mockmeta = $this->createStubAssetMetadata();
+    return $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Aggregate\\AssetAggregate', array($mockmeta));
   }
+
 }
\ No newline at end of file
diff --git a/core/tests/Drupal/Tests/Core/Asset/Collection/AssetCollectionTest.php b/core/tests/Drupal/Tests/Core/Asset/Collection/AssetCollectionTest.php
index d1b2fcc..a4a16ad 100644
--- a/core/tests/Drupal/Tests/Core/Asset/Collection/AssetCollectionTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/Collection/AssetCollectionTest.php
@@ -8,13 +8,13 @@
 namespace Drupal\Tests\Core\Asset\Collection;
 
 use Drupal\Core\Asset\Collection\AssetCollection;
-use Drupal\Tests\Core\Asset\AssetUnitTest;
+use Drupal\Core\Asset\Collection\AssetCollectionBasicInterface;
 
 /**
  * @coversDefaultClass \Drupal\Core\Asset\Collection\AssetCollection
  * @group Asset
  */
-class AssetCollectionTest extends AssetUnitTest {
+class AssetCollectionTest extends BasicAssetCollectionTest {
 
   /**
    * @var AssetCollection
@@ -34,17 +34,26 @@ public function setUp() {
   }
 
   /**
+   * Returns an AssetCollection, the base collection type for this unit test.
+   *
+   * @return AssetCollectionBasicInterface
+   */
+  public function getCollection() {
+    return new AssetCollection();
+  }
+
+  /**
    * @covers ::add
    */
   public function testAdd() {
-    $css = $this->createMockFileAsset('css');
-    $js = $this->createMockFileAsset('js');
+    $asset1 = $this->createStubFileAsset();
+    $asset2 = $this->createStubFileAsset();
 
-    $this->assertTrue($this->collection->add($css));
-    $this->assertTrue($this->collection->add($js));
+    $this->assertTrue($this->collection->add($asset1));
+    $this->assertTrue($this->collection->add($asset2));
 
-    $this->assertContains($css, $this->collection);
-    $this->assertContains($js, $this->collection);
+    $this->assertContains($asset1, $this->collection);
+    $this->assertContains($asset2, $this->collection);
   }
 
   /**
@@ -54,7 +63,7 @@ public function testAdd() {
    * @covers ::add
    */
   public function testDoubleAdd() {
-    $asset = $this->createMockFileAsset('css');
+    $asset = $this->createStubFileAsset();
     $this->assertTrue($this->collection->add($asset));
 
     $this->assertTrue($this->collection->contains($asset));
@@ -75,18 +84,33 @@ public function testDoubleAdd() {
    * @covers ::contains
    */
   public function testContains() {
-    $css = $this->createMockFileAsset('css');
-    $this->collection->add($css);
-    $this->assertTrue($this->collection->contains($css));
+    $asset = $this->createStubFileAsset();
+    $this->collection->add($asset);
+    $this->assertTrue($this->collection->contains($asset));
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testContains
+   * @covers ::__construct
+   */
+  public function testCreateWithAssets() {
+    $asset1 = $this->createStubFileAsset();
+    $asset2 = $this->createStubFileAsset();
+    $collection = new AssetCollection(array($asset1, $asset2));
+
+    $this->assertContains($asset1, $collection);
+    $this->assertContains($asset2, $collection);
   }
 
+
   /**
    * @depends testAdd
    * @covers ::getCss
    */
   public function testGetCss() {
-    $css = $this->createMockFileAsset('css');
-    $js = $this->createMockFileAsset('js');
+    $css = $this->createStubFileAsset('css');
+    $js = $this->createStubFileAsset('js');
 
     $this->collection->add($css);
     $this->collection->add($js);
@@ -104,8 +128,8 @@ public function testGetCss() {
    * @covers ::getJs
    */
   public function testGetJs() {
-    $css = $this->createMockFileAsset('css');
-    $js = $this->createMockFileAsset('js');
+    $css = $this->createStubFileAsset('css');
+    $js = $this->createStubFileAsset('js');
 
     $this->collection->add($css);
     $this->collection->add($js);
@@ -123,8 +147,8 @@ public function testGetJs() {
    * @covers ::all
    */
   public function testAll() {
-    $css = $this->createMockFileAsset('css');
-    $js = $this->createMockFileAsset('js');
+    $css = $this->createStubFileAsset('css');
+    $js = $this->createStubFileAsset('js');
 
     $this->collection->add($css);
     $this->collection->add($js);
@@ -137,7 +161,7 @@ public function testAll() {
    * @covers ::remove
    */
   public function testRemoveByAsset() {
-    $stub = $this->createMockFileAsset('css');
+    $stub = $this->createStubFileAsset();
 
     $this->collection->add($stub);
     $this->collection->remove($stub);
@@ -150,7 +174,7 @@ public function testRemoveByAsset() {
    * @covers ::remove
    */
   public function testRemoveById() {
-    $stub = $this->createMockFileAsset('css');
+    $stub = $this->createStubFileAsset();
 
     $this->collection->add($stub);
     $this->collection->remove($stub->id());
@@ -162,27 +186,17 @@ public function testRemoveById() {
    * @expectedException \OutOfBoundsException
    */
   public function testRemoveNonexistentId() {
-    $this->assertFalse($this->collection->remove('foo'));
-    $this->collection->remove('foo', FALSE);
+    $this->assertFalse($this->collection->remove('foo', TRUE));
+    $this->collection->remove('foo');
   }
 
   /**
    * @expectedException \OutOfBoundsException
    */
   public function testRemoveNonexistentAsset() {
-    $stub = $this->createMockFileAsset('css');
-    $this->assertFalse($this->collection->remove($stub));
-    $this->collection->remove($stub, FALSE);
-  }
-
-  public function testRemoveInvalidType() {
-    $invalid = array(0, 1.1, fopen(__FILE__, 'r'), TRUE, array(), new \stdClass);
-    try {
-      foreach ($invalid as $val) {
-        $this->collection->remove($val);
-        $this->fail('AssetCollection::remove() did not throw exception on invalid argument type.');
-      }
-    } catch (\InvalidArgumentException $e) {}
+    $stub = $this->createStubFileAsset();
+    $this->assertFalse($this->collection->remove($stub, TRUE));
+    $this->collection->remove($stub);
   }
 
   /**
@@ -191,8 +205,8 @@ public function testRemoveInvalidType() {
    */
   public function testMergeCollection() {
     $coll2 = new AssetCollection();
-    $stub1 = $this->createMockFileAsset('css');
-    $stub2 = $this->createMockFileAsset('js');
+    $stub1 = $this->createStubFileAsset();
+    $stub2 = $this->createStubFileAsset();
 
     $coll2->add($stub1);
     $this->collection->mergeCollection($coll2);
@@ -219,7 +233,7 @@ public function testMergeCollection() {
    * correctly trigger an exception.
    */
   public function testExceptionOnWriteWhenFrozen() {
-    $stub = $this->createMockFileAsset('css');
+    $stub = $this->createStubFileAsset();
     $write_protected = array(
       'add' => $stub,
       'remove' => $stub,
@@ -230,10 +244,15 @@ public function testExceptionOnWriteWhenFrozen() {
     foreach ($write_protected as $method => $arg) {
       try {
         $this->collection->$method($arg);
-        $this->fail('Was able to run writable method on frozen AssetCollection');
-      }
-      catch (\LogicException $e) {}
+        $this->fail(sprintf('Was able to run write method "%s" on frozen AssetCollection', $method));
+      } catch (\LogicException $e) {}
     }
+
+    // Do replace method separately, it needs more args
+    try {
+      $this->collection->replace($stub, $this->createStubFileAsset());
+      $this->fail('Was able to run write method "replace" on frozen AssetCollection');
+    } catch (\LogicException $e) {}
   }
 
   /**
@@ -259,18 +278,14 @@ public function testGetById() {
     $this->collection->getById('bar', FALSE);
   }
 
-  public function testIsEmpty() {
-    $this->assertTrue($this->collection->isEmpty());
-  }
-
   /**
    * @depends testAdd
    * @covers ::sort
    */
   public function testSort() {
-    $stub1 = $this->createMockFileAsset('css');
-    $stub2 = $this->createMockFileAsset('js');
-    $stub3 = $this->createMockFileAsset('css');
+    $stub1 = $this->createStubFileAsset();
+    $stub2 = $this->createStubFileAsset();
+    $stub3 = $this->createStubFileAsset();
 
     $this->collection->add($stub1);
     $this->collection->add($stub2);
@@ -296,9 +311,9 @@ public function testSort() {
    * @covers ::ksort
    */
   public function testKsort() {
-    $stub1 = $this->createMockFileAsset('css');
-    $stub2 = $this->createMockFileAsset('js');
-    $stub3 = $this->createMockFileAsset('css');
+    $stub1 = $this->createStubFileAsset();
+    $stub2 = $this->createStubFileAsset();
+    $stub3 = $this->createStubFileAsset();
 
     $this->collection->add($stub1);
     $this->collection->add($stub2);
@@ -315,3 +330,4 @@ public function testKsort() {
     $this->assertEquals($assets, $this->collection->all());
   }
 }
+
diff --git a/core/tests/Drupal/Tests/Core/Asset/Collection/BasicAssetCollectionTest.php b/core/tests/Drupal/Tests/Core/Asset/Collection/BasicAssetCollectionTest.php
new file mode 100644
index 0000000..f657657
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/Collection/BasicAssetCollectionTest.php
@@ -0,0 +1,457 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Asset\Collection\BasicAssetCollectionTest.
+ */
+
+namespace Drupal\Tests\Core\Asset\Collection;
+
+use Drupal\Core\Asset\Collection\AssetCollectionBasicInterface;
+use Drupal\Core\Asset\Collection\BasicAssetCollection;
+use Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException;
+use Drupal\Tests\Core\Asset\AssetUnitTest;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Asset\Collection\BasicAssetCollection
+ * @group Asset
+ */
+class BasicAssetCollectionTest extends AssetUnitTest {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'BasicAssetCollection unit tests',
+      'description' => 'Unit tests for BasicAssetCollection',
+      'group' => 'Asset',
+    );
+  }
+
+  /**
+   * Generates a simple BasicAssetCollection mock.
+   *
+   * @return BasicAssetCollection
+   */
+  public function getBasicCollection() {
+    return $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Collection\\BasicAssetCollection');
+  }
+
+  /**
+   * Method to return the appropriate collection type for the current test.
+   *
+   * @return AssetCollectionBasicInterface
+   */
+  public function getCollection() {
+    return $this->getBasicCollection();
+  }
+
+  /**
+   * Generates a AssetAggregate mock with three leaf assets.
+   */
+  public function getThreeLeafBasicCollection() {
+    $collection = $this->getCollection();
+    $nested_aggregate = $this->getAggregate();
+
+    foreach (array('foo', 'bar', 'baz') as $var) {
+      $$var = $this->createStubFileAsset('css', $var);
+    }
+
+    $nested_aggregate->add($foo);
+    $nested_aggregate->add($bar);
+    $collection->add($nested_aggregate);
+    $collection->add($baz);
+
+    return array($collection, $foo, $bar, $baz, $nested_aggregate);
+  }
+
+  /**
+   * This uses PHPUnit's reflection-based assertions rather than assertContains
+   * so that this test can honestly sit at the root of the test method
+   * dependency tree.
+   *
+   * @covers ::add
+   */
+  public function testAdd() {
+    $collection = $this->getCollection();
+    $asset = $this->createStubFileAsset();
+    $this->assertTrue($collection->add($asset));
+
+    $this->assertAttributeContains($asset, 'assetStorage', $collection);
+    $this->assertAttributeContains($asset, 'assetIdMap', $collection);
+
+    // Nesting: add an aggregate to the first aggregate.
+    $nested_aggregate = $this->getAggregate();
+    $collection->add($nested_aggregate);
+
+    $this->assertAttributeContains($nested_aggregate, 'assetStorage', $collection);
+    $this->assertAttributeContains($nested_aggregate, 'assetIdMap', $collection);
+    $this->assertAttributeContains($nested_aggregate, 'nestedStorage', $collection);
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testEach
+   * @covers ::__construct
+   */
+  public function testCreateWithAssets() {
+    $asset1 = $this->createStubFileAsset();
+    $asset2 = $this->createStubFileAsset();
+    $collection = $this->getMockForAbstractClass('\\Drupal\\Core\\Asset\\Collection\\BasicAssetCollection', array(array($asset1, $asset2)));
+
+    $this->assertContains($asset1, $collection);
+    $this->assertContains($asset2, $collection);
+  }
+
+  /**
+   * @expectedException \Drupal\Core\Asset\Exception\UnsupportedAsseticBehaviorException
+   * @covers ::add
+   */
+  public function testVanillaAsseticAdd() {
+    $vanilla = $this->getMock('\\Assetic\\Asset\\BaseAsset', array(), array(), '', FALSE);
+    $this->getCollection()->add($vanilla);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::each
+   * @covers ::getIterator
+   * @covers \Drupal\Core\Asset\Collection\Iterator\RecursiveBasicCollectionIterator
+   */
+  public function testEach() {
+    list($collection, $foo, $bar, $baz, $nested_aggregate) = $this->getThreeLeafBasicCollection();
+
+    $contained = array();
+    foreach ($collection->each() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($nested_aggregate, $foo, $bar, $baz), $contained);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::eachLeaf
+   * @covers \Drupal\Core\Asset\Collection\Iterator\RecursiveBasicCollectionIterator
+   */
+  public function testEachLeaf() {
+    list($collection, $foo, $bar, $baz) = $this->getThreeLeafBasicCollection();
+
+    $contained = array();
+    foreach ($collection->eachLeaf() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($foo, $bar, $baz), $contained);
+  }
+
+  /**
+   * Tests that adding the same asset twice is disallowed.
+   *
+   * @depends testAdd
+   * @covers ::add
+   */
+  public function testDoubleAdd() {
+    $collection = $this->getCollection();
+    $asset = $this->createStubFileAsset();
+    $this->assertTrue($collection->add($asset));
+
+    // Test by object identity
+    $this->assertFalse($collection->add($asset));
+    // Test by id
+    $asset2 = $this->createStubFileAsset('css', $asset->id());
+
+    $this->assertFalse($collection->add($asset2));
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::contains
+   */
+  public function testContains() {
+    $collection = $this->getCollection();
+    $asset = $this->createStubFileAsset();
+    $collection->add($asset);
+
+    $this->assertTrue($collection->contains($asset));
+
+    // Nesting: add an aggregate to the first aggregate.
+    $nested_aggregate = $this->getAggregate();
+    $nested_asset = $this->createStubFileAsset();
+
+    $nested_aggregate->add($nested_asset);
+    $collection->add($nested_aggregate);
+
+    $this->assertTrue($collection->contains($nested_asset));
+  }
+
+  /**
+   * @covers ::getById
+   * @expectedException \OutOfBoundsException
+   */
+  public function testGetById() {
+    $collection = $this->getCollection();
+
+    $asset = $this->createStubFileAsset();
+    $collection->add($asset);
+    $this->assertSame($asset, $collection->getById($asset->id()));
+
+    $nested_aggregate = $this->getAggregate();
+    $nested_asset = $this->createStubFileAsset();
+
+    $nested_aggregate->add($nested_asset);
+    $collection->add($nested_aggregate);
+
+    $this->assertSame($nested_asset, $collection->getById($nested_asset->id()));
+
+    // Nonexistent asset
+    $this->assertFalse($collection->getById('bar'));
+
+    // Nonexistent asset, non-graceful
+    $collection->getById('bar', FALSE);
+  }
+
+  /**
+   * @depends testAdd
+   * @covers ::all
+   */
+  public function testAll() {
+    $collection = $this->getCollection();
+
+    $asset1 = $this->createStubFileAsset();
+    $asset2 = $this->createStubFileAsset();
+    $collection->add($asset1);
+    $collection->add($asset2);
+
+    $output = array(
+      $asset1->id() => $asset1,
+      $asset2->id() => $asset2,
+    );
+
+    $this->assertEquals($output, $collection->all());
+
+    // Ensure that only top-level assets are returned.
+    $nested_aggregate = $this->getAggregate();
+    $nested_aggregate->add($this->createStubFileAsset());
+    $collection->add($nested_aggregate);
+
+    $output[$nested_aggregate->id()] = $nested_aggregate;
+    $this->assertEquals($output, $collection->all());
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::remove
+   * @covers ::doRemove
+   */
+  public function testRemove() {
+    list($collection, $foo, $bar, $baz, $nested_aggregate) = $this->getThreeLeafBasicCollection();
+    $this->assertFalse($collection->remove('arglebargle', TRUE));
+    $this->assertTrue($collection->remove('foo'));
+
+    $this->assertNotContains($foo, $collection);
+    $this->assertContains($bar, $collection);
+    $this->assertContains($baz, $collection);
+
+    $this->assertTrue($collection->remove($bar));
+
+    $this->assertNotContains($bar, $collection);
+    $this->assertContains($baz, $collection);
+
+    $this->assertTrue($collection->remove($nested_aggregate));
+    $this->assertNotContains($nested_aggregate, $collection);
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::remove
+   * @covers ::doRemove
+   * @expectedException \OutOfBoundsException
+   */
+  public function testRemoveNonexistentNeedle() {
+    list($collection) = $this->getThreeLeafBasicCollection();
+    // Nonexistent leaf removal returns FALSE in graceful mode
+    $this->assertFalse($collection->remove($this->createStubFileAsset(), TRUE));
+
+    // In non-graceful mode, an exception is thrown.
+    $collection->remove($this->createStubFileAsset());
+  }
+
+  /**
+   * @depends testEach
+   * @depends testEachLeaf
+   * @covers ::replace
+   * @covers ::doReplace
+   */
+  public function testReplace() {
+    list($collection, $foo, $bar, $baz, $nested_aggregate) = $this->getThreeLeafBasicCollection();
+    $qux = $this->createStubFileAsset('css', 'qux');
+
+    $this->assertFalse($collection->replace('arglebargle', $qux, TRUE));
+    $this->assertTrue($collection->replace('foo', $qux));
+
+    $this->assertContains($qux, $collection);
+    $this->assertNotContains($foo, $collection);
+
+    $contained = array();
+    foreach ($collection->eachLeaf() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($qux, $bar, $baz), $contained);
+
+    $this->assertTrue($collection->replace($bar, $foo));
+
+    $this->assertContains($foo, $collection);
+    $this->assertNotContains($bar, $collection);
+
+    $contained = array();
+    foreach ($collection->eachLeaf() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($qux, $foo, $baz), $contained);
+
+    $aggregate2 = $this->getAggregate();
+    $this->assertTrue($collection->replace($baz, $aggregate2));
+
+    $this->assertContains($aggregate2, $collection);
+    $this->assertNotContains($baz, $collection);
+
+    $contained = array();
+    foreach ($collection->eachLeaf() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($qux, $foo), $contained);
+
+    $contained = array();
+    foreach ($collection->each() as $leaf) {
+      $contained[] = $leaf;
+    }
+    $this->assertEquals(array($nested_aggregate, $qux, $foo, $aggregate2), $contained);
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::replace
+   * @covers ::doReplace
+   * @expectedException \OutOfBoundsException
+   */
+  public function testReplaceNonexistentNeedle() {
+    list($collection) = $this->getThreeLeafBasicCollection();
+    // Nonexistent leaf replacement returns FALSE in graceful mode
+    $qux = $this->createStubFileAsset();
+    $this->assertFalse($collection->replace($this->createStubFileAsset(), $qux, TRUE));
+    $this->assertNotContains($qux, $collection);
+
+    // In non-graceful mode, an exception is thrown.
+    $collection->replace($this->createStubFileAsset(), $qux);
+  }
+
+  /**
+   * @depends testEach
+   * @covers ::replace
+   * @expectedException \LogicException
+   */
+  public function testReplaceWithAlreadyPresentAsset() {
+    list($aggregate, $foo) = $this->getThreeLeafBasicCollection();
+    $aggregate->replace($this->createStubFileAsset(), $foo);
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testReplaceWithAlreadyPresentAsset
+   * @covers ::replace
+   * @expectedException \LogicException
+   *
+   * This fails on the same check that testReplaceWithAlreadyPresentAsset,
+   * but it is demonstrated as its own test for clarity.
+   */
+  public function testReplaceWithSelf() {
+    list($collection, $foo) = $this->getThreeLeafBasicCollection();
+    $collection->replace($foo, $foo);
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testRemove
+   * @covers ::isEmpty
+   */
+  public function testIsEmpty() {
+    $collection = $this->getCollection();
+    $this->assertTrue($collection->isEmpty());
+
+    // Collections containing only empty collections are considered empty.
+    $collection->add($this->getAggregate());
+    $this->assertTrue($collection->isEmpty());
+
+    $aggregate = $this->getAggregate();
+    $asset = $this->createStubFileAsset();
+    $aggregate->add($asset);
+    $collection->add($aggregate);
+    $this->assertFalse($collection->isEmpty());
+
+    $collection->remove($aggregate);
+    $this->assertTrue($collection->isEmpty());
+
+    $collection->add($asset);
+    $this->assertFalse($collection->isEmpty());
+
+    $collection->remove($asset);
+    $this->assertTrue($collection->isEmpty());
+  }
+
+  /**
+   * @depends testAdd
+   * @depends testRemove
+   * @covers ::count
+   */
+  public function testCount() {
+    $collection = $this->getCollection();
+    $this->assertCount(0, $collection);
+
+    $collection->add($this->getAggregate());
+    $this->assertCount(0, $collection);
+
+    $aggregate = $this->getAggregate();
+    $asset = $this->createStubFileAsset();
+    $aggregate->add($asset);
+    $collection->add($aggregate);
+    $this->assertCount(1, $collection);
+
+    $collection->remove($aggregate);
+    $this->assertCount(0, $collection);
+
+    $collection->add($asset);
+    $this->assertCount(1, $collection);
+
+    $collection->remove($asset);
+    $this->assertCount(0, $collection);
+  }
+
+  /**
+   * @covers ::remove
+   */
+  public function testRemoveInvalidNeedle() {
+    $collection = $this->getCollection();
+    $invalid = array(0, 1.1, fopen(__FILE__, 'r'), TRUE, array(), new \stdClass);
+
+    try {
+      foreach ($invalid as $val) {
+        $collection->remove($val);
+        $this->fail('BasicAssetCollection::remove() did not throw exception on invalid argument type for $needle.');
+      }
+    } catch (\InvalidArgumentException $e) {}
+  }
+
+  /**
+   * @covers ::replace
+   */
+  public function testReplaceInvalidNeedle() {
+    $collection = $this->getCollection();
+    $invalid = array(0, 1.1, fopen(__FILE__, 'r'), TRUE, array(), new \stdClass);
+
+    try {
+      foreach ($invalid as $val) {
+        $collection->replace($val, $this->createStubFileAsset());
+        $this->fail('BasicAssetCollection::replace() did not throw exception on invalid argument type for $needle.');
+      }
+    } catch (\InvalidArgumentException $e) {}
+  }
+
+}
+
diff --git a/core/tests/Drupal/Tests/Core/Asset/Factory/AssetCollectorTest.php b/core/tests/Drupal/Tests/Core/Asset/Factory/AssetCollectorTest.php
index 723de5d..437d09a 100644
--- a/core/tests/Drupal/Tests/Core/Asset/Factory/AssetCollectorTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/Factory/AssetCollectorTest.php
@@ -50,6 +50,19 @@ public function setUp() {
   }
 
   /**
+   * Tests that constructor-injected params end up in the right place.
+   */
+  public function testConstructorInjection() {
+    $factory = $this->getMock('Drupal\\Core\\Asset\\Metadata\\DefaultAssetMetadataFactory');
+    $collection = $this->getMock('Drupal\\Core\\Asset\\Collection\\AssetCollection');
+
+    $collector = new AssetCollector($collection, $factory);
+
+    $this->assertAttributeSame($collection, 'collection', $collector);
+    $this->assertAttributeSame($factory, 'metadataFactory', $collector);
+  }
+
+  /**
    * Tests that the collector injects provided metadata to created assets.
    */
   public function testMetadataInjection() {
@@ -97,7 +110,7 @@ public function testAddAssetExplicitly() {
     $collection = new AssetCollection();
     $this->collector->setCollection($collection);
 
-    $mock = $this->createMockFileAsset('css');
+    $mock = $this->createStubFileAsset('css');
     $this->collector->add($mock);
 
     $this->assertContains($mock, $collection);