diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index c1266d7..9b5c6e8 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\Asset;
 
-use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Cache\CacheCollectorInterface;
 use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
 
@@ -61,14 +60,23 @@ public function __construct(CacheCollectorInterface $library_discovery_collector
    * {@inheritdoc}
    */
   public function getLibrariesByExtension($extension) {
-    return $this->getLibraryDefinitions($extension);
+    if (!isset($this->libraryDefinitions[$extension])) {
+      $libraries = $this->collector->get($extension);
+      $this->libraryDefinitions[$extension] = [];
+      foreach ($libraries as $name => $definition) {
+        $library_name = "$extension/$name";
+        $this->libraryDefinitions[$extension][$name] = $definition;
+      }
+    }
+
+    return $this->libraryDefinitions[$extension];
   }
 
   /**
    * {@inheritdoc}
    */
   public function getLibraryByName($extension, $name) {
-    $extension = $this->getLibraryDefinitions($extension);
+    $extension = $this->getLibrariesByExtension($extension);
     return isset($extension[$name]) ? $extension[$name] : FALSE;
   }
 
@@ -81,47 +89,4 @@ public function clearCachedDefinitions() {
     $this->collector->clear();
   }
 
-  /**
-   * Returns the library definitions for a given extension.
-   *
-   * This also implements libraries-overrides for entire libraries that have
-   * been specified by the LibraryDiscoveryParser.
-   *
-   * @param string $extension
-   *   The name of the extension for which library definitions will be returned.
-   *
-   * @return array
-   *   The library definitions for $extension will overrides applied.
-   */
-  protected function getLibraryDefinitions($extension) {
-    if (!isset($this->libraryDefinitions[$extension])) {
-      $libraries = $this->collector->get($extension);
-      $this->libraryDefinitions[$extension] = [];
-      foreach ($libraries as $name => $definition) {
-        // Handle libraries that are marked for override or removal.
-        // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
-        if (isset($definition['override'])) {
-          if ($definition['override'] === FALSE) {
-            // Remove the library definition if FALSE is given.
-            unset($libraries[$name]);
-          }
-          else {
-            // Otherwise replace with existing library definition if it exists.
-            // Throw an exception if it doesn't.
-            list($replacement_extension, $replacement_name) = explode('/', $definition['override']);
-            $replacement_definition = $this->getLibraryDefinitions($replacement_extension);
-            if (isset($replacement_definition[$replacement_name])) {
-              $libraries[$name] = $replacement_definition[$replacement_name];
-            }
-            else {
-              throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $definition['override']));
-            }
-          }
-        }
-      }
-      $this->libraryDefinitions[$extension] = $libraries;
-    }
-    return $this->libraryDefinitions[$extension];
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
index 7ab83d5..016a498 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Asset;
 
+use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Cache\CacheCollector;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Lock\LockBackendInterface;
@@ -79,9 +80,52 @@ protected function getCid() {
    * {@inheritdoc}
    */
   protected function resolveCacheMiss($key) {
-    $this->storage[$key] = $this->discoveryParser->buildByExtension($key);
+    $this->storage[$key] = $this->getLibraryDefinitions($key);
     $this->persist($key);
 
     return $this->storage[$key];
   }
+
+
+  /**
+   * Returns the library definitions for a given extension.
+   *
+   * This also implements libraries-overrides for entire libraries that have
+   * been specified by the LibraryDiscoveryParser.
+   *
+   * @param string $extension
+   *   The name of the extension for which library definitions will be returned.
+   *
+   * @return array
+   *   The library definitions for $extension will overrides applied.
+   *
+   * @throws \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException
+   */
+  protected function getLibraryDefinitions($extension) {
+    $libraries = $this->discoveryParser->buildByExtension($extension);
+    foreach ($libraries as $name => $definition) {
+      // Handle libraries that are marked for override or removal.
+      // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
+      if (isset($definition['override'])) {
+        if ($definition['override'] === FALSE) {
+          // Remove the library definition if FALSE is given.
+          unset($libraries[$name]);
+        }
+        else {
+          // Otherwise replace with existing library definition if it exists.
+          // Throw an exception if it doesn't.
+          list($replacement_extension, $replacement_name) = explode('/', $definition['override']);
+          $replacement_definition = $this->get($replacement_extension);
+          if (isset($replacement_definition[$replacement_name])) {
+            $libraries[$name] = $replacement_definition[$replacement_name];
+          }
+          else {
+            throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $definition['override']));
+          }
+        }
+      }
+    }
+    return $libraries;
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index 07a2ecb..59e3532 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -11,7 +11,7 @@
 use Drupal\simpletest\KernelTestBase;
 
 /**
- * Tests the element info.
+ * Tests the library discovery and library discovery parser.
  *
  * @group Render
  */
@@ -130,20 +130,20 @@ public function testLibrariesOverrideMalformedAsset() {
    * Tests library assets with other ways for specifying paths.
    */
   public function testLibrariesOverrideOtherAssetLibraryNames() {
-    // Activate test theme that defines libraries overrides on other types of
+    // Activate a test theme that defines libraries overrides on other types of
     // assets.
     $this->activateTheme('test_theme');
 
-    // Assert Drupal relative paths.
+    // Assert Drupal-relative paths.
     $this->assertAssetInLibrary('themes/my_theme/css/dropbutton.css', 'core', 'drupal.dropbutton', 'css');
 
     // Assert stream wrapper paths.
     $this->assertAssetInLibrary('public://my_css/vertical-tabs.css', 'core', 'drupal.vertical-tabs', 'css');
 
-    // Assert protocol-relative URI.
+    // Assert a protocol-relative URI.
     $this->assertAssetInLibrary('//my-server/my_theme/css/jquery_ui.css', 'core', 'jquery.ui', 'css');
 
-    // Assert regular URI.
+    // Assert an absolute URI.
     $this->assertAssetInLibrary('http://example.com/my_theme/css/farbtastic.css', 'core', 'jquery.farbtastic', 'css');
   }
 
@@ -151,11 +151,11 @@ public function testLibrariesOverrideOtherAssetLibraryNames() {
    * Tests that base theme libraries-override still apply in sub themes.
    */
   public function testBaseThemeLibrariesOverrideInSubTheme() {
-    // Activate test theme that has subthemes.
+    // Activate a test theme that has subthemes.
     $this->activateTheme('test_subtheme');
 
     // Assert that libraries-override specified in the base theme still applies
-    // in sub theme.
+    // in the sub theme.
     $this->assertNoAssetInLibrary('core/misc/dialog/dialog.js', 'core', 'drupal.dialog', 'js');
     $this->assertAssetInLibrary('core/modules/system/tests/themes/test_basetheme/css/farbtastic.css', 'core', 'jquery.farbtastic', 'css');
   }
@@ -188,11 +188,11 @@ protected function activateTheme($theme_name) {
    * @param string $asset
    *   The asset file with the path for the file.
    * @param string $extension
-   *   The extension wherein the $library is defined.
+   *   The extension in which the $library is defined.
    * @param string $library_name
    *   Name of the library.
    * @param mixed $sub_key
-   *   The library sub_key where the given asset is defined.
+   *   The library sub key where the given asset is defined.
    * @param string $message
    *   (optional) A message to display with the assertion.
    *
@@ -218,11 +218,11 @@ protected function assertAssetInLibrary($asset, $extension, $library_name, $sub_
    * @param string $asset
    *   The asset file with the path for the file.
    * @param string $extension
-   *   The extension where-in the $library_name is defined.
+   *   The extension in which the $library_name is defined.
    * @param string $library_name
    *   Name of the library.
    * @param mixed $sub_key
-   *   The library sub_key where the given asset is defined.
+   *   The library sub key where the given asset is defined.
    * @param string $message
    *   (optional) A message to display with the assertion.
    *
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index f0525f4..d872e33 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -40,7 +40,7 @@ libraries-override:
   core/jquery:
     js:
       assets/vendor/jquery/jquery.min.js: js/collapse.js
-  # Use Drupal relative paths.
+  # Use Drupal-relative paths.
   core/drupal.dropbutton:
     css:
       component:
@@ -50,12 +50,12 @@ libraries-override:
     css:
       component:
         misc/vertical-tabs.css: public://my_css/vertical-tabs.css
-  # Use protocol-relative URI.
+  # Use a protocol-relative URI.
   core/jquery.ui:
     css:
       component:
         assets/vendor/jquery.ui/themes/base/core.css: //my-server/my_theme/css/jquery_ui.css
-  # Use regular URI.
+  # Use an absolute URI.
   core/jquery.farbtastic:
     css:
       component:
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDependencyResolverTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDependencyResolverTest.php
index 5ab278a..e8af816 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDependencyResolverTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDependencyResolverTest.php
@@ -60,10 +60,10 @@ class LibraryDependencyResolverTest extends UnitTestCase {
   protected function setUp() {
     $this->libraryDiscovery = $this->getMockBuilder('Drupal\Core\Asset\LibraryDiscovery')
       ->disableOriginalConstructor()
-      ->setMethods(['getLibraryDefinitions'])
+      ->setMethods(['getLibrariesByExtension'])
       ->getMock();
     $this->libraryDiscovery->expects($this->any())
-      ->method('getLibraryDefinitions')
+      ->method('getLibrariesByExtension')
       ->with('test')
       ->will($this->returnValue($this->libraryData));
     $this->libraryDependencyResolver= new LibraryDependencyResolver($this->libraryDiscovery);