diff --git a/core/core.services.yml b/core/core.services.yml
index efa4b34..2ae0391 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -476,11 +476,13 @@ services:
     arguments: ['@app.root', '@module_handler', '@kernel', '@router.builder']
     lazy: true
   extension.list.module:
-    class: Drupal\Core\Extension\ModuleExtensionList
-    arguments: ['@app.root', 'module', '@cache.default', '@info_parser', '@module_handler', '@config.factory', '@extension.list.profile']
+    class: Drupal\Core\Extension\Hub\ExtensionHubInterface
+    factory: Drupal\Core\Extension\Hub\ExtensionHub::createForModules
+    arguments: ['@app.root', '@info_parser', '@module_handler', '@config.factory', '@extension.list.profile', '@cache.default']
   extension.list.profile:
-    class: Drupal\Core\Extension\ProfileExtensionList
-    arguments: ['@app.root', 'profile', '@cache.default', '@info_parser', '@module_handler']
+    class: Drupal\Core\Extension\Hub\ExtensionHubInterface
+    factory: Drupal\Core\Extension\Hub\ExtensionHub::createForProfiles
+    arguments: ['@app.root', '@info_parser', '@module_handler', '@cache.default']
   content_uninstall_validator:
     class: Drupal\Core\Entity\ContentUninstallValidator
     tags:
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 5f5a19a..76c1be7 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -206,12 +206,12 @@ function drupal_get_filename($type, $name, $filename = NULL) {
     $extension_list = \Drupal::service($service_id);
     if (isset($filename)) {
       // Manually add the info file path of an extension.
-      $extension_list->setFilename($name, $filename);
+      $extension_list->nameSetFilename($name, $filename);
       return;
     }
     else {
       try {
-        return $extension_list->getFilename($name);
+        return $extension_list->nameGetFilename($name);
       }
       catch (\InvalidArgumentException $e) {
         // Catch the exception and trigger error to maintain existing behavior.
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 50108fa..f35d0f2 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -618,7 +618,7 @@ function drupal_install_system($install_state) {
   // Before having installed the system module and being able to do a module
   // rebuild, prime the ModuleExtensionList static cache with the module's
   // exact location.
-  \Drupal::service('extension.list.module')->setFilename('system', 'core/modules/system/system.info.yml');
+  \Drupal::service('extension.list.module')->nameSetFilename('system', 'core/modules/system/system.info.yml');
 
   // Install base system configuration.
   \Drupal::service('config.installer')->installDefaultConfig('core', 'core');
diff --git a/core/lib/Drupal/Core/Extension/Extension.php b/core/lib/Drupal/Core/Extension/Extension.php
index 0238714..2c3cf81 100644
--- a/core/lib/Drupal/Core/Extension/Extension.php
+++ b/core/lib/Drupal/Core/Extension/Extension.php
@@ -45,6 +45,13 @@ class Extension implements \Serializable {
   protected $root;
 
   /**
+   * Parsed contents of the *.info.yml file. Stays null until initialized.
+   *
+   * @var array|null
+   */
+  public $info;
+
+  /**
    * Constructs a new Extension object.
    *
    * @param string $root
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index cdee878..cdbd521 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -18,7 +18,7 @@
  * to your settings.php.
  *
  */
-class ExtensionDiscovery {
+class ExtensionDiscovery implements ExtensionDiscoveryInterface {
 
   /**
    * Origin directory weight: Core.
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscoveryInterface.php b/core/lib/Drupal/Core/Extension/ExtensionDiscoveryInterface.php
new file mode 100644
index 0000000..24af3c8
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscoveryInterface.php
@@ -0,0 +1,58 @@
+<?php
+namespace Drupal\Core\Extension;
+
+/**
+ * Discovers available extensions in the filesystem.
+ *
+ * To also discover test modules, add
+ *
+ * @code
+ * $settings['extension_discovery_scan_tests'] = TRUE;
+ * @encode
+ * to your settings.php.
+ *
+ */
+interface ExtensionDiscoveryInterface {
+
+  /**
+   * Discovers available extensions of a given type.
+   *
+   * Finds all extensions (modules, themes, etc) that exist on the site. It
+   * searches in several locations. For instance, to discover all available
+   * modules:
+   *
+   * @code
+   * $listing = new ExtensionDiscovery(\Drupal::root());
+   * $modules = $listing->scan('module');
+   * @endcode
+   *
+   * The following directories will be searched (in the order stated):
+   * - the core directory; i.e., /core
+   * - the installation profile directory; e.g., /core/profiles/standard
+   * - the legacy site-wide directory; i.e., /sites/all
+   * - the site-wide directory; i.e., /
+   * - the site-specific directory; e.g., /sites/example.com
+   *
+   * To also find test modules, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
+   * The information is returned in an associative array, keyed by the extension
+   * name (without .info.yml extension). Extensions found later in the search
+   * will take precedence over extensions found earlier - unless they are not
+   * compatible with the current version of Drupal core.
+   *
+   * @param string $type
+   *   The extension type to search for. One of 'profile', 'module', 'theme', or
+   *   'theme_engine'.
+   * @param bool $include_tests
+   *   (optional) Whether to explicitly include or exclude test extensions. By
+   *   default, test extensions are only discovered when in a test environment.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   *   An associative array of Extension objects, keyed by extension name.
+   */
+  public function scan($type, $include_tests = NULL);
+}
diff --git a/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
deleted file mode 100644
index 925d319..0000000
--- a/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ /dev/null
@@ -1,467 +0,0 @@
-<?php
-
-namespace Drupal\Core\Extension;
-
-use Drupal\Core\Cache\CacheBackendInterface;
-
-/**
- * Provides available extensions.
- *
- * The extension list is per extension type, like module theme and profile.
- */
-abstract class ExtensionList {
-
-  /**
-   * The type of the extension, such as "module" or "theme".
-   *
-   * @var string
-   */
-  protected $type;
-
-  /**
-   * The app root.
-   *
-   * @var string
-   */
-  protected $root;
-
-  /**
-   * The cache.
-   *
-   * @var \Drupal\Core\Cache\CacheBackendInterface
-   */
-  protected $cache;
-
-  /**
-   * Default values to be merged into *.info.yml file arrays.
-   *
-   * @var mixed[]
-   */
-  protected $defaults = [];
-
-  /**
-   * The info parser.
-   *
-   * @var \Drupal\Core\Extension\InfoParserInterface
-   */
-  protected $infoParser;
-
-  /**
-   * The module handler.
-   *
-   * @var \Drupal\Core\Extension\ModuleHandlerInterface
-   */
-  protected $moduleHandler;
-
-  /**
-   * The statically cached extensions.
-   *
-   * @var \Drupal\Core\Extension\Extension[]
-   */
-  protected $extensions;
-
-  /**
-   * Static caching for extension info.
-   *
-   * @var array[]
-   *   Keys are extension names, and values their info arrays (mixed[]).
-   */
-  protected $extensionInfo;
-
-  /**
-   * A list of extension folder names keyed by extension name.
-   *
-   * @var string[]
-   */
-  protected $fileNames;
-
-  /**
-   * A list of extension folder names directly added in code (not discovered).
-   *
-   * It is important to keep a separate list to ensure that it takes priority
-   * over the discovered extension folders.
-   *
-   * @var string[]
-   */
-  protected $addedFileNames;
-
-  /**
-   * The extension discovery service.
-   *
-   * @var \Drupal\Core\Extension\ExtensionDiscovery
-   */
-  protected $extensionDiscovery;
-
-  /**
-   * Constructs a new ExtensionList instance.
-   *
-   * @param string $root
-   *   The app root.
-   * @param string $type
-   *   The extension type.
-   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
-   *   The cache.
-   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
-   *   The info parser.
-   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
-   *   The module handler.
-   */
-  public function __construct($root, $type, CacheBackendInterface $cache, InfoParserInterface $info_parser, ModuleHandlerInterface $module_handler) {
-    $this->root = $root;
-    $this->type = $type;
-    $this->cache = $cache;
-    $this->infoParser = $info_parser;
-    $this->moduleHandler = $module_handler;
-    $this->extensionDiscovery = $this->getExtensionDiscovery();
-  }
-
-  /**
-   * Returns the extension discovery.
-   *
-   * @return \Drupal\Core\Extension\ExtensionDiscovery
-   */
-  protected function getExtensionDiscovery() {
-    return new ExtensionDiscovery($this->root);
-  }
-
-  /**
-   * Resets the stored extension list.
-   */
-  public function reset() {
-    $this->extensions = NULL;
-    $this->cache->delete($this->getListCacheId());
-    $this->extensionInfo = NULL;
-    $this->cache->delete($this->getInfoCacheId());
-    $this->fileNames = NULL;
-    $this->cache->delete($this->getFilenameCacheId());
-    $this->addedFileNames = NULL;
-    return $this;
-  }
-
-  /**
-   * Returns the extension list cache ID.
-   *
-   * @return string
-   */
-  protected function getListCacheId() {
-    return 'core.extension_listing.' . $this->type;
-  }
-
-  /**
-   * Returns the extension info cache ID.
-   *
-   * @return string
-   */
-  protected function getInfoCacheId() {
-    return "system.{$this->type}.info";
-  }
-
-  /**
-   * Returns the extension filenames cache ID.
-   *
-   * @return string
-   */
-  protected function getFilenameCacheId() {
-    return "system.{$this->type}.files";
-  }
-
-  /**
-   * Determines if an extension exists in the filesystem.
-   *
-   * @param string $name
-   *   The machine name of the extension.
-   *
-   * @return bool
-   *   True if the extension exists (whether installed or not) and false if not.
-   */
-  public function extensionExists($name) {
-    $extensions = $this->listExtensions();
-    return isset($extensions[$name]);
-  }
-
-  /**
-   * Returns the human readable name of the extension.
-   *
-   * @param string $machine_name
-   *   The extension name.
-   *
-   * @return string
-   *   The human readable name of the extension.
-   *
-   * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied machine name.
-   */
-  public function getName($machine_name) {
-    return $this->getExtension($machine_name)->info['name'];
-  }
-
-  /**
-   * Returns a single extension.
-   *
-   * @param string $name
-   *   The extension name.
-   *
-   * @return \Drupal\Core\Extension\Extension
-   *
-   * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied name.
-   */
-  public function getExtension($name) {
-    $extensions = $this->listExtensions();
-    if (isset($extensions[$name])) {
-      return $extensions[$name];
-    }
-
-    throw new \InvalidArgumentException("The {$this->type} $name does not exist.");
-  }
-
-  /**
-   * Returns all available extensions.
-   *
-   * @return \Drupal\Core\Extension\Extension[]
-   */
-  public function listExtensions() {
-    if (isset($this->extensions)) {
-      return $this->extensions;
-    }
-    if ($cache = $this->cache->get($this->getListCacheId())) {
-      $this->extensions = $cache->data;
-      return $this->extensions;
-    }
-    $extensions = $this->doListExtensions();
-    $this->cache->set($this->getListCacheId(), $extensions);
-    $this->extensions = $extensions;
-    return $this->extensions;
-  }
-
-  /**
-   * Scans the available extensions.
-   *
-   * Overriding this method gives other code the chance to add additional
-   * extensions to this raw listing.
-   *
-   * @return \Drupal\Core\Extension\Extension[]
-   */
-  protected function doScanExtensions() {
-    return $this->extensionDiscovery->scan($this->type);
-  }
-
-  /**
-   * Build the actual list of extensions before caching it.
-   *
-   * @return \Drupal\Core\Extension\Extension[]
-   */
-  protected function doListExtensions() {
-    // Find extensions.
-    $extensions = $this->doScanExtensions();
-
-    // Read info files for each extension.
-    foreach ($extensions as $name => $extension) {
-      // Look for the info file.
-      $extension->info = $this->infoParser->parse($extension->getPathname());
-
-      // Add the info file modification time, so it becomes available for
-      // contributed extensions to use for ordering extension lists.
-      $extension->info['mtime'] = $extension->getMTime();
-
-      // Merge in defaults and save.
-      $extensions[$name]->info = $extension->info + $this->defaults;
-
-      // Invoke hook_system_info_alter() to give installed modules a chance to
-      // modify the data in the .info.yml files if necessary.
-      $this->moduleHandler->alter('system_info', $extensions[$name]->info, $extensions[$name], $this->type);
-    }
-
-    return $extensions;
-  }
-
-  /**
-   * Returns information about a specified extension.
-   *
-   * This function returns the contents of the .info.yml file for the specified
-   * installed extension.
-   *
-   * @param string $name
-   *   The name of an extension whose information shall be returned. If
-   *   $name does not exist or is not enabled, an empty array will be returned.
-   *
-   * @return mixed[]
-   *   An associative array of extension information.
-   *
-   * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied name.
-   */
-  public function getInfo($name) {
-    // Ensure that $this->extensionInfo is primed.
-    $this->getAllInfo();
-    if (isset($this->extensionInfo[$name])) {
-      return $this->extensionInfo[$name];
-    }
-    throw new \InvalidArgumentException("The {$this->type} $name does not exist.");
-  }
-
-  /**
-   * Returns an array of information about enabled modules or themes.
-   *
-   * This function returns the contents of the .info.yml file for each installed
-   * extension.
-   *
-   * @return array[]
-   *   An associative array of extension information keyed by name. If no
-   *   records are available, an empty array is returned.
-   */
-  public function getAllInfo() {
-    if (!isset($this->extensionInfo)) {
-      $cache_key = "system.{$this->type}.info";
-      if ($cache = $this->cache->get($cache_key)) {
-        $info = $cache->data;
-      }
-      else {
-        $info = $this->recalculateInfo();
-        $this->cache->set($cache_key, $info);
-      }
-      $this->extensionInfo = $info;
-    }
-    return $this->extensionInfo;
-  }
-
-  /**
-   * Generates the information from .info.yml files for extensions of this type.
-   *
-   * The information is placed in cache with the "system.{extension_type}.info"
-   * key.
-   *
-   * @return array[]
-   *   An array of arrays of .info.yml entries keyed by the extension name.
-   */
-  protected function recalculateInfo() {
-    $info = [];
-    foreach ($this->listExtensions() as $name => $extension) {
-      $info[$name] = $extension->info;
-    }
-    return $info;
-  }
-
-  /**
-   * Returns a list of extension folder names keyed by extension name.
-   *
-   * @return string[]
-   */
-  public function getFilenames() {
-    if (!isset($this->fileNames)) {
-      $cache_id = $this->getFilenameCacheId();
-      if ($cache = $this->cache->get($cache_id)) {
-        $file_names = $cache->data;
-      }
-      else {
-        $file_names = $this->recalculateFilenames();
-        $this->cache->set($cache_id, $file_names);
-      }
-      $this->fileNames = $file_names;
-    }
-    return $this->fileNames;
-  }
-
-  /**
-   * Generates a sorted list of .info.yml file locations for all extensions.
-   *
-   * The information is placed in cache with the "system.{extension_type}.files"
-   * key.
-   *
-   * @return string[]
-   *   An array of .info.yml file locations keyed by the extension name.
-   */
-  protected function recalculateFilenames() {
-    $file_names = [];
-    $extensions = $this->listExtensions();
-    ksort($extensions);
-    foreach ($extensions as $name => $extension) {
-      $file_names[$name] = $extension->getPathname();
-    }
-    return $file_names;
-  }
-
-  /**
-   * Sets the filename for an extension.
-   *
-   * This method is used in the Drupal bootstrapping phase, when the extension
-   * system is not fully initialized, to manually set locations of modules and
-   * profiles needed to complete bootstrapping.
-   *
-   * It is not recommended to call this method except in those rare cases.
-   *
-   * @param string $extension_name
-   *   The name of the extension for which the filename is requested.
-   * @param string $filename
-   *   The filename of the extension which is to be set explicitly rather
-   *   than by consulting the dynamic extension listing.
-   */
-  public function setFilename($extension_name, $filename) {
-    $this->addedFileNames[$extension_name] = $filename;
-  }
-
-  /**
-   * Gets the filename for a system resource.
-   *
-   * The filename, whether provided, cached, or retrieved from the database, is
-   * only returned if the file exists.
-   *
-   * This function plays a key role in allowing Drupal's extensions (modules,
-   * themes, profiles, theme_engines, etc.) to be located in different places
-   * depending on a site's configuration. For example, a module 'foo' may
-   * legally be located in any of these three places:
-   *
-   * core/modules/foo/foo.info.yml
-   * modules/foo/foo.info.yml
-   * sites/all/modules/foo/foo.info.yml
-   * sites/example.com/modules/foo/foo.info.yml
-   *
-   * while a theme 'bar' may be located in any of similar places:
-   *
-   * core/themes/bar/bar.info.yml
-   * themes/bar/bar.info.yml
-   * sites/all/themes/bar/bar.info.yml
-   * sites/example.com/themes/bar/bar.info.yml
-   *
-   * Calling ExtensionList::getFilename('foo') will give you one of the above,
-   * depending on where the extension is located and what type it is.
-   *
-   * @param string $extension_name
-   *   The name of the extension for which the filename is requested.
-   *
-   * @return string
-   *   The filename of the requested extension's .info.yml file.
-   *
-   * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied name.
-   */
-  public function getFilename($extension_name) {
-    // Ensure that $this->fileNames is primed.
-    $this->getFilenames();
-    if (isset($this->addedFileNames[$extension_name])) {
-      return $this->addedFileNames[$extension_name];
-    }
-    elseif (isset($this->fileNames[$extension_name])) {
-      return $this->fileNames[$extension_name];
-    }
-    throw new \InvalidArgumentException("The {$this->type} $extension_name does not exist.");
-  }
-
-
-  /**
-   * Gets the path to an extension of a specific type (module, theme, etc.).
-   *
-   * @param string $extension_name
-   *   The name of the extension for which the path is requested.
-   *
-   * @return string
-   *   The drupal-root-relative path to the specified extension.
-   *
-   * @throws \InvalidArgumentException
-   *   If there is no extension with the supplied name.
-   */
-  public function getPath($extension_name) {
-    return dirname($this->getFilename($extension_name));
-  }
-
-}
diff --git a/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameList.php b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameList.php
new file mode 100644
index 0000000..1d537ca
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameList.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\Core\Extension\FilenameList;
+
+use Drupal\Core\Extension\List_\ExtensionListInterface;
+
+class ExtensionFilenameList implements ExtensionFilenameListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  private $extensionList;
+
+  /**
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $extension_list
+   */
+  function __construct(ExtensionListInterface $extension_list) {
+    $this->extensionList = $extension_list;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFilenames() {
+    $file_names = [];
+    $extensions = $this->extensionList->listExtensions();
+    ksort($extensions);
+    foreach ($extensions as $name => $extension) {
+      $file_names[$name] = $extension->getPathname();
+    }
+    return $file_names;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListBuffer.php b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListBuffer.php
new file mode 100644
index 0000000..ab5887b
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListBuffer.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\Core\Extension\FilenameList;
+
+class ExtensionFilenameListBuffer implements ExtensionFilenameListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface
+   */
+  private $decorated;
+
+  /**
+   * @var string[]|null
+   */
+  private $fileNames;
+
+  /**
+   * @param \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface $decorated
+   */
+  function __construct(ExtensionFilenameListInterface $decorated) {
+    $this->decorated = $decorated;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->fileNames = NULL;
+    $this->decorated->reset();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFilenames() {
+    return $this->fileNames !== NULL
+      ? $this->fileNames
+      : $this->fileNames = $this->decorated->getFilenames();
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListCache.php b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListCache.php
new file mode 100644
index 0000000..b1da4c3
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListCache.php
@@ -0,0 +1,59 @@
+<?php
+
+namespace Drupal\Core\Extension\FilenameList;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+
+/**
+ * Cache decorator for extension file list.
+ */
+class ExtensionFilenameListCache implements ExtensionFilenameListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface
+   */
+  private $decorated;
+
+  /**
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  private $cache;
+
+  /**
+   * @var string
+   */
+  private $cacheId;
+
+  /**
+   * @param \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface $decorated
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   * @param string $cache_id
+   */
+  function __construct(ExtensionFilenameListInterface $decorated, CacheBackendInterface $cache, $cache_id) {
+    $this->decorated = $decorated;
+    $this->cache = $cache;
+    $this->cacheId = $cache_id;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->cache->delete($this->cacheId);
+    $this->decorated->reset();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFilenames() {
+    if ($cache = $this->cache->get($this->cacheId)) {
+      return $cache->data;
+    }
+    // Resolve cache miss.
+    $file_names = $this->decorated->getFilenames();
+    $this->cache->set($this->cacheId, $file_names);
+    return $file_names;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListInterface.php b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListInterface.php
new file mode 100644
index 0000000..b2ac3a4
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/FilenameList/ExtensionFilenameListInterface.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\Core\Extension\FilenameList;
+
+interface ExtensionFilenameListInterface {
+
+  /**
+   * Resets cached or buffered extension file names.
+   *
+   * @return $this
+   */
+  public function reset();
+
+  /**
+   * Returns a list of extension folder names keyed by extension name.
+   *
+   * @return string[]
+   */
+  public function getFilenames();
+
+}
diff --git a/core/lib/Drupal/Core/Extension/Hub/ExtensionHub.php b/core/lib/Drupal/Core/Extension/Hub/ExtensionHub.php
new file mode 100644
index 0000000..2e5bd6e
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/Hub/ExtensionHub.php
@@ -0,0 +1,257 @@
+<?php
+
+namespace Drupal\Core\Extension\Hub;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\FilenameList\ExtensionFilenameList;
+use Drupal\Core\Extension\FilenameList\ExtensionFilenameListBuffer;
+use Drupal\Core\Extension\FilenameList\ExtensionFilenameListCache;
+use Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface;
+use Drupal\Core\Extension\InfoList\ExtensionInfoList;
+use Drupal\Core\Extension\InfoList\ExtensionInfoListBuffer;
+use Drupal\Core\Extension\InfoList\ExtensionInfoListCache;
+use Drupal\Core\Extension\InfoList\ExtensionInfoListInterface;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\List_\ExtensionListBuffer;
+use Drupal\Core\Extension\List_\ExtensionListCache;
+use Drupal\Core\Extension\List_\ExtensionListInterface;
+use Drupal\Core\Extension\List_\ModuleDiscoveryExtensionList;
+use Drupal\Core\Extension\List_\ProfileDiscoveryExtensionList;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+
+/**
+ * Default implementation for ExtensionHubInterface.
+ */
+class ExtensionHub implements ExtensionHubInterface {
+
+  /**
+   * The type of the extension, such as "module" or "theme".
+   *
+   * @var string
+   */
+  private $type;
+
+  /**
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  private $source;
+
+  /**
+   * @var \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface
+   */
+  private $infoList;
+
+  /**
+   * @var \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface
+   */
+  private $fileList;
+
+  /**
+   * A list of extension file names directly added in code (not discovered).
+   *
+   * It is important to keep a separate list to ensure that it takes priority
+   * over the discovered extension folders.
+   *
+   * @var string[]
+   */
+  private $addedFileNames = [];
+
+  /**
+   * @param string $root
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $profile_list
+   *
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   *
+   * @return \Drupal\Core\Extension\Hub\ExtensionHubInterface
+   */
+  static function createForModules(
+    $root,
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ConfigFactoryInterface $config_factory,
+    ExtensionListInterface $profile_list,
+    CacheBackendInterface $cache
+  ) {
+    return self::create(
+      'module',
+      ModuleDiscoveryExtensionList::create(
+        $info_parser,
+        $module_handler,
+        new ExtensionDiscovery($root),
+        $config_factory,
+        $profile_list),
+      $cache);
+  }
+
+  /**
+   * @param $root
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   *
+   * @return \Drupal\Core\Extension\Hub\ExtensionHubInterface
+   */
+  static function createForProfiles(
+    $root,
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    CacheBackendInterface $cache
+  ) {
+    return self::create(
+      'profile',
+      ProfileDiscoveryExtensionList::create(
+        $info_parser,
+        $module_handler,
+        new ExtensionDiscovery($root)),
+      $cache);
+  }
+
+  /**
+   * @param string $type
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $extension_list
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   *
+   * @return \Drupal\Core\Extension\Hub\ExtensionHubInterface
+   */
+  static function create(
+    $type,
+    ExtensionListInterface $extension_list,
+    CacheBackendInterface $cache
+  ) {
+
+    $extension_list = new ExtensionListCache($extension_list, $cache, 'core.extension.list.' . $type);
+    $extension_list = new ExtensionListBuffer($extension_list);
+
+    $info_list = new ExtensionInfoList($extension_list);
+    $info_list = new ExtensionInfoListCache($info_list, $cache, "system.$type.info");
+    $info_list = new ExtensionInfoListBuffer($info_list);
+
+    $file_list = new ExtensionFilenameList($extension_list);
+    $file_list = new ExtensionFilenameListCache($file_list, $cache, "system.$type.files");
+    $file_list = new ExtensionFilenameListBuffer($file_list);
+
+    return new self($type, $extension_list, $info_list, $file_list);
+  }
+
+  /**
+   * @param string $type
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $source
+   * @param \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface $info_list
+   * @param \Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface $file_list
+   */
+  function __construct(
+    $type,
+    ExtensionListInterface $source,
+    ExtensionInfoListInterface $info_list,
+    ExtensionFilenameListInterface $file_list
+  ) {
+    $this->type = $type;
+    $this->source = $source;
+    $this->infoList = $info_list;
+    $this->fileList = $file_list;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->source->reset();
+    $this->infoList->reset();
+    $this->fileList->reset();
+    $this->addedFileNames = NULL;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function extensionExists($name) {
+    $extensions = $this->listExtensions();
+    return isset($extensions[$name]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameGetLabel($machine_name) {
+    return $this->nameGetExtension($machine_name)->info['name'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameGetExtension($name) {
+    $extensions = $this->listExtensions();
+    if (isset($extensions[$name])) {
+      return $extensions[$name];
+    }
+
+    throw new \InvalidArgumentException("The {$this->type} $name does not exist.");
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function listExtensions() {
+    return $this->source->listExtensions();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameGetInfo($extension_name) {
+    // Ensure that $this->extensionInfo is primed.
+    $allInfo = $this->getAllInfo();
+    if (isset($allInfo[$extension_name])) {
+      return $allInfo[$extension_name];
+    }
+    throw new \InvalidArgumentException("The {$this->type} $extension_name does not exist.");
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllInfo() {
+    return $this->infoList->getAllInfo();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFilenames() {
+    return $this->fileList->getFilenames();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameSetFilename($extension_name, $filename) {
+    $this->addedFileNames[$extension_name] = $filename;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameGetFilename($extension_name) {
+    if (isset($this->addedFileNames[$extension_name])) {
+      return $this->addedFileNames[$extension_name];
+    }
+    $filenames = $this->getFilenames();
+    if (isset($filenames[$extension_name])) {
+      return $filenames[$extension_name];
+    }
+    throw new \InvalidArgumentException("The {$this->type} $extension_name does not exist.");
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function nameGetPath($extension_name) {
+    return dirname($this->nameGetFilename($extension_name));
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/Hub/ExtensionHubInterface.php b/core/lib/Drupal/Core/Extension/Hub/ExtensionHubInterface.php
new file mode 100644
index 0000000..5001aac
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/Hub/ExtensionHubInterface.php
@@ -0,0 +1,144 @@
+<?php
+namespace Drupal\Core\Extension\Hub;
+
+use Drupal\Core\Extension\FilenameList\ExtensionFilenameListInterface;
+use Drupal\Core\Extension\InfoList\ExtensionInfoListInterface;
+use Drupal\Core\Extension\List_\ExtensionListInterface;
+
+/**
+ * Provides various information about available extensions, for a specific
+ * extension type (e.g. 'module', 'theme' or 'profile').
+ */
+interface ExtensionHubInterface extends ExtensionListInterface, ExtensionFilenameListInterface, ExtensionInfoListInterface {
+
+  /**
+   * Resets any stored or buffered data.
+   *
+   * @return $this
+   */
+  public function reset();
+
+  /**
+   * Determines if an extension exists in the filesystem.
+   *
+   * @param string $name
+   *   The machine name of the extension.
+   *
+   * @return bool
+   *   True if the extension exists (whether installed or not) and false if not.
+   */
+  public function extensionExists($name);
+
+  /**
+   * Returns the human readable name of the extension.
+   *
+   * @param string $machine_name
+   *   The extension name.
+   *
+   * @return string
+   *   The human readable name of the extension.
+   *
+   * @throws \InvalidArgumentException
+   *   If there is no extension with the supplied machine name.
+   */
+  public function nameGetLabel($machine_name);
+
+  /**
+   * Returns a single extension.
+   *
+   * @param string $name
+   *   The extension machine name.
+   *
+   * @return \Drupal\Core\Extension\Extension
+   *
+   * @throws \InvalidArgumentException
+   *   If there is no extension with the supplied name.
+   */
+  public function nameGetExtension($name);
+
+  /**
+   * Returns information about a specified extension.
+   *
+   * This function returns the contents of the .info.yml file for the specified
+   * installed extension.
+   *
+   * @param string $extension_name
+   *   The name of an extension whose information shall be returned. If an
+   *   extension with this name does not exist, an exception is thrown.
+   *
+   * @return mixed[]
+   *   An associative array of extension information.
+   *
+   * @throws \InvalidArgumentException
+   *   If there is no extension with the supplied name.
+   */
+  public function nameGetInfo($extension_name);
+
+  /**
+   * Sets the filename for an extension.
+   *
+   * This method is used in the Drupal bootstrapping phase, when the extension
+   * system is not fully initialized, to manually set locations of modules and
+   * profiles needed to complete bootstrapping.
+   *
+   * It is not recommended to call this method except in those rare cases.
+   *
+   * @param string $extension_name
+   *   The name of the extension for which the filename is requested.
+   * @param string $filename
+   *   The filename of the extension which is to be set explicitly rather
+   *   than by consulting the dynamic extension listing.
+   */
+  public function nameSetFilename($extension_name, $filename);
+
+  /**
+   * Gets the filename for a system resource.
+   *
+   * The filename, whether provided, cached, or retrieved from the database, is
+   * only returned if the file exists.
+   *
+   * This function plays a key role in allowing Drupal's extensions (modules,
+   * themes, profiles, theme_engines, etc.) to be located in different places
+   * depending on a site's configuration. For example, a module 'foo' may
+   * legally be located in any of these three places:
+   *
+   * core/modules/foo/foo.info.yml
+   * modules/foo/foo.info.yml
+   * sites/all/modules/foo/foo.info.yml
+   * sites/example.com/modules/foo/foo.info.yml
+   *
+   * while a theme 'bar' may be located in any of similar places:
+   *
+   * core/themes/bar/bar.info.yml
+   * themes/bar/bar.info.yml
+   * sites/all/themes/bar/bar.info.yml
+   * sites/example.com/themes/bar/bar.info.yml
+   *
+   * Calling ExtensionList::getFilename('foo') will give you one of the above,
+   * depending on where the extension is located and what type it is.
+   *
+   * @param string $extension_name
+   *   The name of the extension for which the filename is requested.
+   *
+   * @return string
+   *   The filename of the requested extension's .info.yml file.
+   *
+   * @throws \InvalidArgumentException
+   *   If there is no extension with the supplied name.
+   */
+  public function nameGetFilename($extension_name);
+
+  /**
+   * Gets the path to an extension of a specific type (module, theme, etc.).
+   *
+   * @param string $extension_name
+   *   The name of the extension for which the path is requested.
+   *
+   * @return string
+   *   The drupal-root-relative path to the specified extension.
+   *
+   * @throws \InvalidArgumentException
+   *   If there is no extension with the supplied name.
+   */
+  public function nameGetPath($extension_name);
+}
diff --git a/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoList.php b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoList.php
new file mode 100644
index 0000000..66693da
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoList.php
@@ -0,0 +1,38 @@
+<?php
+
+namespace Drupal\Core\Extension\InfoList;
+
+use Drupal\Core\Extension\List_\ExtensionListInterface;
+
+class ExtensionInfoList implements ExtensionInfoListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  private $extensionList;
+
+  /**
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $extension_list
+   */
+  function __construct(ExtensionListInterface $extension_list) {
+    $this->extensionList = $extension_list;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllInfo() {
+    $info = [];
+    foreach ($this->extensionList->listExtensions() as $name => $extension) {
+      $info[$name] = $extension->info;
+    }
+    return $info;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListBuffer.php b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListBuffer.php
new file mode 100644
index 0000000..27245d3
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListBuffer.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\Core\Extension\InfoList;
+
+class ExtensionInfoListBuffer implements ExtensionInfoListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface
+   */
+  private $decorated;
+
+  /**
+   * @var array[]|null
+   */
+  private $extensionInfo;
+
+  /**
+   * @param \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface $decorated
+   */
+  function __construct(ExtensionInfoListInterface $decorated) {
+    $this->decorated = $decorated;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->extensionInfo = NULL;
+    $this->decorated->reset();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllInfo() {
+    return $this->extensionInfo !== NULL
+      ? $this->extensionInfo
+      : $this->extensionInfo = $this->decorated->getAllInfo();
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListCache.php b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListCache.php
new file mode 100644
index 0000000..d8458c6
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListCache.php
@@ -0,0 +1,56 @@
+<?php
+
+namespace Drupal\Core\Extension\InfoList;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+
+class ExtensionInfoListCache implements ExtensionInfoListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface
+   */
+  private $decorated;
+
+  /**
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  private $cache;
+
+  /**
+   * @var string
+   */
+  private $cache_id;
+
+  /**
+   * @param \Drupal\Core\Extension\InfoList\ExtensionInfoListInterface $decorated
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   * @param string $cache_id
+   */
+  function __construct(ExtensionInfoListInterface $decorated, CacheBackendInterface $cache, $cache_id) {
+    $this->decorated = $decorated;
+    $this->cache = $cache;
+    $this->cache_id = $cache_id;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->cache->delete($this->cache_id);
+    $this->decorated->reset();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllInfo() {
+    if ($cache = $this->cache->get($this->cache_id)) {
+      return $cache->data;
+    }
+    // Resolve cache miss.
+    $info = $this->decorated->getAllInfo();
+    $this->cache->set($this->cache_id, $info);
+    return $info;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListInterface.php b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListInterface.php
new file mode 100644
index 0000000..0c57d93
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/InfoList/ExtensionInfoListInterface.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\Core\Extension\InfoList;
+
+interface ExtensionInfoListInterface {
+
+  /**
+   * Resets cached or buffered extension info.
+   *
+   * @return $this
+   */
+  public function reset();
+
+  /**
+   * Returns an array of information about enabled modules or themes.
+   *
+   * This function returns the contents of the .info.yml file for each installed
+   * extension.
+   *
+   * @return array[]
+   *   An associative array of extension information keyed by name. If no
+   *   records are available, an empty array is returned.
+   */
+  public function getAllInfo();
+
+}
diff --git a/core/lib/Drupal/Core/Extension/List_/DiscoveryExtensionListBase.php b/core/lib/Drupal/Core/Extension/List_/DiscoveryExtensionListBase.php
new file mode 100644
index 0000000..1188ef3
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/List_/DiscoveryExtensionListBase.php
@@ -0,0 +1,127 @@
+<?php
+
+namespace Drupal\Core\Extension\List_;
+
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+
+abstract class DiscoveryExtensionListBase implements ExtensionListInterface {
+
+  /**
+   * The type of the extension, such as "module" or "theme".
+   *
+   * @var string
+   */
+  private $type;
+
+  /**
+   * The info parser.
+   *
+   * @var \Drupal\Core\Extension\InfoParserInterface
+   */
+  private $infoParser;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The extension discovery service.
+   *
+   * @var \Drupal\Core\Extension\ExtensionDiscovery
+   */
+  protected $extensionDiscovery;
+
+  /**
+   * Default values to be merged into *.info.yml file arrays.
+   *
+   * @var mixed[]
+   */
+  private $defaults;
+
+  /**
+   * Constructs a new DiscoveryExtensionListSource instance.
+   *
+   * @param string $type
+   *   The extension type.
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   *   The info parser.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   * @param \Drupal\Core\Extension\ExtensionDiscovery $extension_discovery
+   *   The extension discovery object.
+   * @param array $info_defaults
+   *   Defaults to merge into $extension->info.
+   */
+  public function __construct(
+    $type,
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ExtensionDiscovery $extension_discovery,
+    array $info_defaults
+  ) {
+    $this->type = $type;
+    $this->infoParser = $info_parser;
+    $this->moduleHandler = $module_handler;
+    $this->extensionDiscovery = $extension_discovery;
+    $this->defaults = $info_defaults;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    // Nothing to do.
+  }
+
+  /**
+   * Scans the available extensions.
+   *
+   * Overriding this method gives other code the chance to add additional
+   * extensions to this raw listing.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   */
+  protected function doScanExtensions() {
+    return $this->extensionDiscovery->scan($this->type);
+  }
+
+  /**
+   * Returns all available extensions.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   *
+   * @throws \Drupal\Core\Extension\InfoParserException
+   *   If one of the .info.yml is broken or incomplete.
+   */
+  public function listExtensions() {
+    // Find extensions.
+    $extensions = $this->doScanExtensions();
+
+    // Read info files for each extension.
+    foreach ($extensions as $name => $extension) {
+      // @todo Clone the extension object, to prevent side effects?
+      
+      // Look for the info file.
+      $extension->info = $this->infoParser->parse($extension->getPathname());
+
+      // Add the info file modification time, so it becomes available for
+      // contributed extensions to use for ordering extension lists.
+      /** @noinspection PhpUndefinedMethodInspection */
+      $extension->info['mtime'] = $extension->getMTime();
+
+      // Merge in defaults and save.
+      $extension->info += $this->defaults;
+
+      // Invoke hook_system_info_alter() to give installed modules a chance to
+      // modify the data in the .info.yml files if necessary.
+      $this->moduleHandler->alter('system_info', $extensions[$name]->info, $extensions[$name], $this->type);
+    }
+
+    return $extensions;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/List_/ExtensionListBuffer.php b/core/lib/Drupal/Core/Extension/List_/ExtensionListBuffer.php
new file mode 100644
index 0000000..1132ee9
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/List_/ExtensionListBuffer.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\Core\Extension\List_;
+
+class ExtensionListBuffer implements ExtensionListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  private $decorated;
+
+  /**
+   * @var \Drupal\Core\Extension\Extension[]|null
+   */
+  private $extensions;
+
+  /**
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $decorated
+   */
+  function __construct(ExtensionListInterface $decorated) {
+    $this->decorated = $decorated;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->decorated->reset();
+    $this->extensions = NULL;
+    return $this;
+  }
+
+  /**
+   * Returns all available extensions, with $extension->info filled in.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   */
+  public function listExtensions() {
+    return NULL !== $this->extensions
+      ? $this->extensions
+      : $this->extensions = $this->decorated->listExtensions();
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/List_/ExtensionListCache.php b/core/lib/Drupal/Core/Extension/List_/ExtensionListCache.php
new file mode 100644
index 0000000..7e10e9c
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/List_/ExtensionListCache.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace Drupal\Core\Extension\List_;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+
+class ExtensionListCache implements ExtensionListInterface {
+
+  /**
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  private $decorated;
+
+  /**
+   * The cache.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  private $cache;
+
+  /**
+   * @var string
+   */
+  private $cache_id;
+
+  /**
+   * Constructs a new ExtensionList instance.
+   *
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $decorated
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   * @param string $cache_id
+   */
+  public function __construct(ExtensionListInterface $decorated, CacheBackendInterface $cache, $cache_id) {
+    $this->cache = $cache;
+    $this->decorated = $decorated;
+    $this->cache_id = $cache_id;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->cache->delete($this->cache_id);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function listExtensions() {
+    if ($cache = $this->cache->get($this->cache_id)) {
+      return $cache->data;
+    }
+    $extensions = $this->decorated->listExtensions();
+    $this->cache->set($this->cache_id, $extensions);
+    return $extensions;
+  }
+}
diff --git a/core/lib/Drupal/Core/Extension/List_/ExtensionListInterface.php b/core/lib/Drupal/Core/Extension/List_/ExtensionListInterface.php
new file mode 100644
index 0000000..df866c1
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/List_/ExtensionListInterface.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\Core\Extension\List_;
+
+interface ExtensionListInterface {
+
+  /**
+   * Resets any stored or cached extension list.
+   *
+   * @return $this
+   */
+  public function reset();
+
+  /**
+   * Returns all available extensions, with $extension->info filled in.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   */
+  public function listExtensions();
+
+}
diff --git a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php b/core/lib/Drupal/Core/Extension/List_/ModuleDiscoveryExtensionList.php
similarity index 63%
rename from core/lib/Drupal/Core/Extension/ModuleExtensionList.php
rename to core/lib/Drupal/Core/Extension/List_/ModuleDiscoveryExtensionList.php
index 4becd70..b37ce36 100644
--- a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/List_/ModuleDiscoveryExtensionList.php
@@ -1,63 +1,81 @@
 <?php
 
-namespace Drupal\Core\Extension;
+namespace Drupal\Core\Extension\List_;
 
-use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\ModuleHandler;
+use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 
-/**
- * Provides a list of available modules.
- */
-class ModuleExtensionList extends ExtensionList {
+class ModuleDiscoveryExtensionList extends DiscoveryExtensionListBase {
 
+  // @todo Use an injected (lazy/proxy) translation service.
   use StringTranslationTrait;
 
   /**
-   * {@inheritdoc}
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
    */
-  protected $defaults = [
-    'dependencies' => [],
-    'description' => '',
-    'package' => 'Other',
-    'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
-  ];
+  private $configFactory;
 
   /**
-   * The config factory.
-   *
-   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   * @var \Drupal\Core\Extension\List_\ExtensionListInterface
    */
-  protected $configFactory;
+  private $profileList;
 
   /**
-   * The profile list needed by this module list.
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Extension\ExtensionDiscovery $extension_discovery
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $profile_list
    *
-   * @var \Drupal\Core\Extension\ExtensionList
+   * @return \Drupal\Core\Extension\List_\ModuleDiscoveryExtensionList
    */
-  protected $profileList;
+  static function create(
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ExtensionDiscovery $extension_discovery,
+    ConfigFactoryInterface $config_factory,
+    ExtensionListInterface $profile_list
+  ) {
+    return new self(
+      'module',
+      $info_parser,
+      $module_handler,
+      $extension_discovery,
+      [
+        'dependencies' => [],
+        'description' => '',
+        'package' => 'Other',
+        'version' => NULL,
+        'php' => DRUPAL_MINIMUM_PHP,
+      ],
+      $config_factory,
+      $profile_list);
+  }
 
   /**
-   * Constructs a new ModuleExtensionList instance.
-   *
-   * @param string $root
-   *   The app root.
    * @param string $type
-   *   The extension type.
-   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
-   *   The cache.
    * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
-   *   The info parser.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
-   *   The module handler.
+   * @param \Drupal\Core\Extension\ExtensionDiscovery $extension_discovery
+   * @param array $info_defaults
    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
-   *   The config factory.
-   * @param \Drupal\Core\Extension\ExtensionList $profile_list
-   *   The site profile listing.
+   * @param \Drupal\Core\Extension\List_\ExtensionListInterface $profile_list
    */
-  public function __construct($root, $type, CacheBackendInterface $cache, InfoParserInterface $info_parser, ModuleHandlerInterface $module_handler, ConfigFactoryInterface $config_factory, ExtensionList $profile_list) {
-    parent::__construct($root, $type, $cache, $info_parser, $module_handler);
+  function __construct(
+    $type,
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ExtensionDiscovery $extension_discovery,
+    array $info_defaults,
+    ConfigFactoryInterface $config_factory,
+    ExtensionListInterface $profile_list
+  ) {
+    parent::__construct($type, $info_parser, $module_handler, $extension_discovery, $info_defaults);
 
     $this->configFactory = $config_factory;
     $this->profileList = $profile_list;
@@ -85,7 +103,7 @@ protected function doScanExtensions() {
   /**
    * {@inheritdoc}
    */
-  protected function doListExtensions() {
+  public function listExtensions() {
     // Find installation profiles. This needs to happen before performing a
     // module scan as the module scan needs to know what the active profile is.
     $profiles = $this->profileList->listExtensions();
@@ -99,7 +117,7 @@ protected function doListExtensions() {
     }
 
     // Find modules.
-    $extensions = parent::doListExtensions();
+    $extensions = parent::listExtensions();
     // It is possible that a module was marked as required by
     // hook_system_info_alter() and modules that it depends on are not required.
     foreach ($extensions as $extension) {
@@ -128,8 +146,11 @@ protected function doListExtensions() {
     // Add status, weight, and schema version.
     $installed_modules = $this->configFactory->get('core.extension')->get('module') ?: [];
     foreach ($extensions as $name => $module) {
+      /** @noinspection PhpUndefinedFieldInspection */
       $module->weight = isset($installed_modules[$name]) ? $installed_modules[$name] : 0;
+      /** @noinspection PhpUndefinedFieldInspection */
       $module->status = (int) isset($installed_modules[$name]);
+      /** @noinspection PhpUndefinedFieldInspection */
       $module->schema_version = SCHEMA_UNINSTALLED;
     }
     $extensions = $this->moduleHandler->buildModuleDependencies($extensions);
@@ -151,6 +172,7 @@ protected function ensureRequiredDependencies(Extension $module, array $modules
         $dependency_name = ModuleHandler::parseDependency($dependency)['name'];
         if (!isset($modules[$dependency_name]->info['required'])) {
           $modules[$dependency_name]->info['required'] = TRUE;
+          // @todo Use an injected (lazy/proxy) translation service.
           $modules[$dependency_name]->info['explanation'] = $this->t('Dependency of required module @module', array('@module' => $module->info['name']));
           // Ensure any dependencies it has are required.
           $this->ensureRequiredDependencies($modules[$dependency_name], $modules);
diff --git a/core/lib/Drupal/Core/Extension/List_/ProfileDiscoveryExtensionList.php b/core/lib/Drupal/Core/Extension/List_/ProfileDiscoveryExtensionList.php
new file mode 100644
index 0000000..06d903d
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/List_/ProfileDiscoveryExtensionList.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\Core\Extension\List_;
+
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+
+class ProfileDiscoveryExtensionList extends DiscoveryExtensionListBase {
+
+  /**
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Extension\ExtensionDiscovery $extension_discovery
+   *
+   * @return \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  static function create(
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ExtensionDiscovery $extension_discovery
+  ) {
+    return new self(
+      'profile',
+      $info_parser,
+      $module_handler,
+      $extension_discovery,
+      [
+        'dependencies' => [],
+        'description' => '',
+        'package' => 'Other',
+        'version' => NULL,
+        'php' => DRUPAL_MINIMUM_PHP,
+      ]);
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandler.php b/core/lib/Drupal/Core/Extension/ModuleHandler.php
index c41a650..5316166 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandler.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandler.php
@@ -717,7 +717,7 @@ public function getModuleDirectories() {
    * {@inheritdoc}
    */
   public function getName($module) {
-    return \Drupal::service('extension.list.module')->getName($module);
+    return \Drupal::service('extension.list.module')->nameGetLabel($module);
   }
 
 }
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
index 1cc2d35..b04a22c 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -308,7 +308,7 @@ public function getModuleDirectories();
    *   in if no matching module is found.
    *
    * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
-   *   Use \Drupal::service('extension.list.module')->getName() instead.
+   *   Use \Drupal::service('extension.list.module')->nameGetLabel() instead.
    */
   public function getName($module);
 
diff --git a/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
index 73064ef..e2ba5ab 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -166,7 +166,7 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
             $module_filenames[$name] = $current_module_filenames[$name];
           }
           else {
-            $module_path = \Drupal::service('extension.list.module')->getPath($name);
+            $module_path = \Drupal::service('extension.list.module')->nameGetPath($name);
             $pathname = "$module_path/$name.info.yml";
             $filename = file_exists($module_path . "/$name.module") ? "$name.module" : NULL;
             $module_filenames[$name] = new Extension($this->root, 'module', $pathname, $filename);
diff --git a/core/lib/Drupal/Core/Extension/ProfileExtensionList.php b/core/lib/Drupal/Core/Extension/ProfileExtensionList.php
deleted file mode 100644
index 256f416..0000000
--- a/core/lib/Drupal/Core/Extension/ProfileExtensionList.php
+++ /dev/null
@@ -1,21 +0,0 @@
-<?php
-
-namespace Drupal\Core\Extension;
-
-/**
- * Provides a list of installation profiles.
- */
-class ProfileExtensionList extends ExtensionList {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected $defaults = [
-    'dependencies' => [],
-    'description' => '',
-    'package' => 'Other',
-    'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
-  ];
-
-}
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 436321d..b286e37 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -916,7 +916,7 @@ function system_check_directory($form_element, FormStateInterface $form_state) {
 function system_get_info($type, $name = NULL) {
   if ($type == 'module') {
     if (isset($name)) {
-      return \Drupal::service('extension.list.module')->getInfo($name);
+      return \Drupal::service('extension.list.module')->nameGetInfo($name);
     }
     else {
       return \Drupal::service('extension.list.module')->getAllInfo();
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionHubTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionHubTest.php
new file mode 100644
index 0000000..198996a
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionHubTest.php
@@ -0,0 +1,237 @@
+<?php
+
+namespace Drupal\Tests\Core\Extension;
+
+use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Extension\Hub\ExtensionHub;
+use Drupal\Core\Extension\InfoParserInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Extension\List_\DiscoveryExtensionListBase;
+use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
+use Prophecy\Argument;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Extension\Hub\ExtensionHub
+ * @group Extension
+ */
+class ExtensionHubTest extends UnitTestCase {
+
+  /**
+   * @covers ::nameGetLabel
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetNameWithNonExistingExtension() {
+    $test_extension_hub = $this->setupEmptyTestExtensionHub();
+
+    $test_extension_hub->nameGetLabel('test_name');
+  }
+
+  /**
+   * @covers ::nameGetLabel
+   */
+  public function testGetName() {
+    $test_extension_hub = $this->setupTestExtensionHub();
+
+    static::assertEquals('test name', $test_extension_hub->nameGetLabel('test_name'));
+  }
+
+  /**
+   * @covers ::nameGetExtension
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetExtensionWithNonExistingExtension() {
+    $test_extension_hub = $this->setupEmptyTestExtensionHub();
+
+    $test_extension_hub->nameGetExtension('test_name');
+  }
+
+  /**
+   * @covers ::nameGetExtension
+   */
+  public function testGetExtension() {
+    $test_extension_hub = $this->setupTestExtensionHub();
+
+    $extension = $test_extension_hub->nameGetExtension('test_name');
+    static::assertInstanceOf(Extension::class, $extension);
+    static::assertEquals('test_name', $extension->getName());
+  }
+
+  /**
+   * @covers ::listExtensions
+   */
+  public function testListExtensions() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $extensions = $test_extension_list->listExtensions();
+    static::assertCount(1, $extensions);
+    static::assertEquals('test_name', $extensions['test_name']->getName());
+  }
+
+  /**
+   * @covers ::nameGetInfo
+   * @covers ::getAllInfo
+   */
+  public function testGetInfo() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $info = $test_extension_list->nameGetInfo('test_name');
+    static::assertEquals([
+      'type' => 'test_extension',
+      'core' => '8.x',
+      'name' => 'test name',
+      'mtime' => 123456789,
+    ], $info);
+  }
+
+  /**
+   * @covers ::getAllInfo
+   */
+  public function testGetAllInfo() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $infos = $test_extension_list->getAllInfo();
+    static::assertEquals(['test_name' => [
+      'type' => 'test_extension',
+      'core' => '8.x',
+      'name' => 'test name',
+      'mtime' => 123456789,
+    ]], $infos);
+  }
+
+  /**
+   * @covers ::getFilenames
+   */
+  public function testGetFilenames() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $filenames = $test_extension_list->getFilenames();
+    static::assertEquals([
+      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
+    ], $filenames);
+  }
+
+  /**
+   * @covers ::nameGetFilename
+   */
+  public function testGetFilename() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $filename = $test_extension_list->nameGetFilename('test_name');
+    static::assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $filename);
+  }
+
+
+  /**
+   * @covers ::nameSetFilename
+   * @covers ::nameGetFilename
+   */
+  public function testSetFilename() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $test_extension_list->nameSetFilename('test_name', 'vfs://drupal_root/example2/test_name/test_name.info.yml');
+    static::assertEquals('vfs://drupal_root/example2/test_name/test_name.info.yml', $test_extension_list->nameGetFilename('test_name'));
+  }
+
+  /**
+   * @covers ::nameGetPath
+   */
+  public function testGetPath() {
+    $test_extension_list = $this->setupTestExtensionHub();
+
+    $path = $test_extension_list->nameGetPath('test_name');
+    static::assertEquals('vfs://drupal_root/example/test_name', $path);
+  }
+
+  /**
+   * @return \Drupal\Core\Extension\Hub\ExtensionHubInterface
+   */
+  protected function setupEmptyTestExtensionHub() {
+    list($cache, $info_parser, $module_handler) = $this->getMocks();
+
+    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
+    $extension_discovery->scan('test_extension')->willReturn([]);
+
+    return ExtensionHub::create(
+      'test_extension',
+      TestExtensionList::create(
+        'test_extension',
+        $info_parser->reveal(),
+        $module_handler->reveal(),
+        $extension_discovery->reveal()),
+      $cache->reveal());
+  }
+
+  /**
+   * @return \Drupal\Core\Extension\Hub\ExtensionHubInterface
+   */
+  protected function setupTestExtensionHub() {
+    vfsStream::setup('drupal_root');
+    vfsStream::create([
+      'example' => [
+        'test_name' => [
+          'test_name.info.yml' => Yaml::encode([
+            'name' => 'test name',
+            'type' => 'test_extension',
+            'core' => '8.x',
+          ]),
+        ],
+      ],
+    ]);
+    touch('vfs://drupal_root/example/test_name/test_name.info.yml', 123456789);
+
+    list($cache, $info_parser, $module_handler) = $this->getMocks();
+    $info_parser->parse(Argument::any())->will(function($args) {
+      return Yaml::decode(file_get_contents($args[0]));
+    });
+
+    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
+    $extension_discovery->scan('test_extension')->willReturn(['test_name' => new Extension($this->root, 'test_extension', 'vfs://drupal_root/example/test_name/test_name.info.yml')]);
+
+    return ExtensionHub::create(
+      'test_extension',
+      TestExtensionList::create(
+        'test_extension',
+        $info_parser->reveal(),
+        $module_handler->reveal(),
+        $extension_discovery->reveal()),
+      $cache->reveal());
+  }
+
+  protected function getMocks() {
+    $cache = $this->prophesize(CacheBackendInterface::class);
+    $info_parser = $this->prophesize(InfoParserInterface::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+    return [$cache, $info_parser, $module_handler];
+  }
+
+}
+
+class TestExtensionList extends DiscoveryExtensionListBase {
+
+  /**
+   * @param string $type
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Extension\ExtensionDiscovery $extension_discovery
+   *
+   * @return \Drupal\Core\Extension\List_\ExtensionListInterface
+   */
+  static function create(
+    $type,
+    InfoParserInterface $info_parser,
+    ModuleHandlerInterface $module_handler,
+    ExtensionDiscovery $extension_discovery
+  ) {
+    return new self(
+      $type,
+      $info_parser,
+      $module_handler,
+      $extension_discovery,
+      []);
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
deleted file mode 100644
index 3ddf93b..0000000
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
+++ /dev/null
@@ -1,204 +0,0 @@
-<?php
-
-namespace Drupal\Tests\Core\Extension;
-
-use Drupal\Component\Serialization\Yaml;
-use Drupal\Core\Cache\CacheBackendInterface;
-use Drupal\Core\Extension\Extension;
-use Drupal\Core\Extension\ExtensionDiscovery;
-use Drupal\Core\Extension\ExtensionList;
-use Drupal\Core\Extension\InfoParserInterface;
-use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\Tests\UnitTestCase;
-use org\bovigo\vfs\vfsStream;
-use Prophecy\Argument;
-
-/**
- * @coversDefaultClass \Drupal\Core\Extension\ExtensionList
- * @group Extension
- */
-class ExtensionListTest extends UnitTestCase {
-
-  /**
-   * @covers ::getName
-   * @expectedException \InvalidArgumentException
-   */
-  public function testGetNameWithNonExistingExtension() {
-    list($cache, $info_parser, $module_handler) = $this->getMocks();
-    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
-
-    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
-    $extension_discovery->scan('test_extension')->willReturn([]);
-    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
-
-    $test_extension_list->getName('test_name');
-  }
-
-  /**
-   * @covers ::getName
-   */
-  public function testGetName() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $this->assertEquals('test name', $test_extension_list->getName('test_name'));
-  }
-
-  /**
-   * @covers ::getExtension
-   * @expectedException \InvalidArgumentException
-   */
-  public function testGetExtensionWithNonExistingExtension() {
-    list($cache, $info_parser, $module_handler) = $this->getMocks();
-    $test_extension_list = new TestExtension($this->root, 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
-
-    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
-    $extension_discovery->scan('test_extension')->willReturn([]);
-    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
-
-    $test_extension_list->getExtension('test_name');
-  }
-
-  /**
-   * @covers ::getExtension
-   */
-  public function testGetExtension() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $extension = $test_extension_list->getExtension('test_name');
-    $this->assertInstanceOf(Extension::class, $extension);
-    $this->assertEquals('test_name', $extension->getName());
-  }
-
-  /**
-   * @covers ::listExtensions
-   */
-  public function testListExtensions() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $extensions = $test_extension_list->listExtensions();
-    $this->assertCount(1, $extensions);
-    $this->assertEquals('test_name', $extensions['test_name']->getName());
-  }
-
-  /**
-   * @covers ::getInfo
-   * @covers ::getAllInfo
-   */
-  public function testGetInfo() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $info = $test_extension_list->getInfo('test_name');
-    $this->assertEquals([
-      'type' => 'test_extension',
-      'core' => '8.x',
-      'name' => 'test name',
-      'mtime' => 123456789,
-    ], $info);
-  }
-
-  /**
-   * @covers ::getAllInfo
-   */
-  public function testGetAllInfo() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $infos = $test_extension_list->getAllInfo();
-    $this->assertEquals(['test_name' => [
-      'type' => 'test_extension',
-      'core' => '8.x',
-      'name' => 'test name',
-      'mtime' => 123456789,
-    ]], $infos);
-  }
-
-  /**
-   * @covers ::getFilenames
-   */
-  public function testGetFilenames() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $filenames = $test_extension_list->getFilenames();
-    $this->assertEquals([
-      'test_name' => 'vfs://drupal_root/example/test_name/test_name.info.yml',
-    ], $filenames);
-  }
-
-  /**
-   * @covers ::getFilename
-   */
-  public function testGetFilename() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $filename = $test_extension_list->getFilename('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $filename);
-  }
-
-
-  /**
-   * @covers ::setFilename
-   * @covers ::getFilename
-   */
-  public function testSetFilename() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $test_extension_list->setFilename('test_name', 'vfs://drupal_root/example2/test_name/test_name.info.yml');
-    $this->assertEquals('vfs://drupal_root/example2/test_name/test_name.info.yml', $test_extension_list->getFilename('test_name'));
-  }
-
-  /**
-   * @covers ::getPath
-   */
-  public function testGetPath() {
-    $test_extension_list = $this->setupTestExtensionList();
-
-    $path = $test_extension_list->getPath('test_name');
-    $this->assertEquals('vfs://drupal_root/example/test_name', $path);
-  }
-
-  /**
-   * @return \Drupal\Tests\Core\Extension\TestExtension
-   */
-  protected function setupTestExtensionList() {
-    vfsStream::setup('drupal_root');
-    vfsStream::create([
-      'example' => [
-        'test_name' => [
-          'test_name.info.yml' => Yaml::encode([
-            'name' => 'test name',
-            'type' => 'test_extension',
-            'core' => '8.x',
-          ]),
-        ],
-      ],
-    ]);
-    touch('vfs://drupal_root/example/test_name/test_name.info.yml', 123456789);
-
-    list($cache, $info_parser, $module_handler) = $this->getMocks();
-    $info_parser->parse(Argument::any())->will(function($args) {
-      return Yaml::decode(file_get_contents($args[0]));
-    });
-
-    $test_extension_list = new TestExtension('vfs://drupal_root', 'test_extension', $cache->reveal(), $info_parser->reveal(), $module_handler->reveal());
-
-    $extension_discovery = $this->prophesize(ExtensionDiscovery::class);
-    $extension_discovery->scan('test_extension')->willReturn(['test_name' => new Extension($this->root, 'test_extension', 'vfs://drupal_root/example/test_name/test_name.info.yml')]);
-    $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
-    return $test_extension_list;
-  }
-
-  protected function getMocks() {
-    $cache = $this->prophesize(CacheBackendInterface::class);
-    $info_parser = $this->prophesize(InfoParserInterface::class);
-    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
-    return [$cache, $info_parser, $module_handler];
-  }
-
-}
-
-class TestExtension extends ExtensionList {
-
-  public function setExtensionDiscovery(ExtensionDiscovery $extension_discovery) {
-    $this->extensionDiscovery = $extension_discovery;
-  }
-
-}