diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 6db3321..cea15c1 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -187,6 +187,13 @@ public function buildByExtension($extension) {
             elseif ($this->fileValidUri($source)) {
               $options['data'] = $source;
             }
+            // A regular URI (e.g., http://example.com/example.js) without
+            // 'external' explicitly specified, which may happen if, e.g.
+            // libraries-override is used.
+            else if ($this->isValidUri($source)) {
+              $options['type'] = 'external';
+              $options['data'] = $source;
+            }
             // By default, file paths are relative to the registering extension.
             else {
               $options['data'] = $path . '/' . $source;
@@ -355,17 +362,17 @@ protected function applyLibrariesOverride($libraries, $extension) {
           list(, , $sub_key, $value) = explode('/', $asset, 4);
           if ($sub_key === 'drupalSettings') {
             // drupalSettings may not be overridden.
-            throw new \LogicException(SafeMarkup::format('drupalSettings may not be overridden in libraries-override. Trying to override %asset. Use hook_library_info_alter() instead.', ['%asset' =>$asset]));
+            throw new \LogicException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', $asset));
           }
           else if ($sub_key === 'css') {
             // SMACSS category should be incorporated into the asset name.
             list($category, $value) = explode('/', $value, 2);
             $parents = [$sub_key, $category, $value];
-            $new_parents = [$sub_key, $category, '/' . $theme_path . '/' . $override];
+            $new_parents = [$sub_key, $category, $override];
           }
           else {
             $parents = [$sub_key, $value];
-            $new_parents = [$sub_key, '/' . $theme_path . '/' . $override];
+            $new_parents = [$sub_key, $override];
           }
           // Remove asset to be overridden, but keep its attributes.
           $attributes = NestedArray::getValue($libraries[$name], $parents);
@@ -394,4 +401,11 @@ protected function fileValidUri($source) {
     return file_valid_uri($source);
   }
 
+  /**
+   * Determines if the supplied string is a valid URI.
+   */
+  protected function isValidUri($string) {
+    return count(explode('://', $string)) === 2;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index d953d49..8779f98 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -173,7 +173,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     foreach ($base_themes as $base) {
       if (!empty($base->info['libraries-override'])) {
         foreach ($base->info['libraries-override'] as $library => $override) {
-          $values['libraries_override'][$library] = $override;
+          $values['libraries_override'][$library] = $override ? $this->resolveThemeAssetPath($base, $library, $override) : $override;
         }
       }
     }
@@ -181,7 +181,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     // Add libraries-override declared by this theme.
     if (!empty($theme->info['libraries-override'])) {
       foreach ($theme->info['libraries-override'] as $library => $override) {
-        $values['libraries_override'][$library] = $override;
+        $values['libraries_override'][$library] = $override ? $this->resolveThemeAssetPath($theme, $library, $override) : $override;
       }
     }
 
@@ -294,4 +294,38 @@ protected function prepareStylesheetsRemove(Extension $theme, $base_themes) {
     return $stylesheets_remove;
   }
 
+  /**
+   * Ensures that a full path is returned for an overriding theme asset.
+   * 
+   * @param \Drupal\Core\Extension\Extension $theme
+   *   The theme or base theme.
+   * @param string $source
+   *   The source asset path, i.e. the library to be replaced.
+   * @param string $destination
+   *   The library or library asset replacing the source.
+   *
+   * @return string
+   */
+  protected function resolveThemeAssetPath(Extension $theme, $source, $destination) {
+    // Full library definitions don't need to be resolved.
+    if (count(explode('/', $source)) === 2) {
+      return $destination;
+    }
+    // The destination is not an absolute path and it's not a URI (e.g.
+    // public://generated_js/example.js or http://example.com/js/my_js.js), so
+    // it's relative to the theme.
+    if ($destination[0] !== '/' && !$this->isValidUri($destination)) {
+      $theme_path = $theme->getPath();
+      return '/' . $theme_path . '/' . $destination;
+    }
+    return $destination;
+  }
+
+  /**
+   * Determines if the supplied string is a valid URI.
+   */
+  protected function isValidUri($string) {
+    return count(explode('://', $string)) === 2;
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index c46ae22..f433390 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -107,7 +107,7 @@ public function testLibrariesOverrideDrupalSettings() {
       $this->fail('Throw LogicException when trying to override drupalSettings');
     }
     catch (\LogicException $e) {
-      $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override <em class="placeholder">core/drupal.ajax/drupalSettings/ajaxPageState</em>. Use hook_library_info_alter() instead.';
+      $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override core/drupal.ajax/drupalSettings/ajaxPageState. Use hook_library_info_alter() instead.';
       $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when trying to override drupalSettings');
     }
   }
@@ -135,15 +135,75 @@ public function testLibrariesOverrideMalformedAsset() {
       $this->fail('Throw LogicException when specifying invalid override');
     }
     catch (\LogicException $e) {
-      $expected_message = 'Library asset <em class="placeholder">core/drupal.dialog/css</em> is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".';
+      $expected_message = 'Library asset core/drupal.dialog/css is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".';
       $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when specifying invalid override');
     }
   }
 
   /**
+   * Tests library assets with edge-casey paths.
+   */
+  public function testLibrariesOverrideOtherAssetLibraryNames() {
+    $this->container->get('theme_handler')->install(['test_theme']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme'));
+
+    // Assert Drupal relative paths.
+    $library = $library_discovery->getLibraryByName('core', 'jquery.once'); 
+    $this->assertAssetInLibrary($library['js'], 'jquery.once', 'themes/my_theme/js/jquery.once.js');
+
+    // Assert streamwrappers.
+    $library = $library_discovery->getLibraryByName('core', 'drupal.active-link'); 
+    $this->assertAssetInLibrary($library['js'], 'drupal.active-link', 'public://my_js/active-link.js');
+
+    // Assert protocol-free URI.
+    $library = $library_discovery->getLibraryByName('core', 'drupal.dialog'); 
+    $this->assertAssetInLibrary($library['js'], 'drupal.dialog', '//my-server/my_theme/js/dialog.js');
+
+    // Assert regular URI.
+    $library = $library_discovery->getLibraryByName('core', 'jquery.farbtastic'); 
+    $this->assertAssetInLibrary($library['js'], 'jquery.farbtastic', 'http://example.com/my_theme/js/farbtastic.js');
+  }
+
+  /**
+   * Tests that base theme libraries-remove still apply in sub themes.
+   */
+  public function testBaseThemeLibrariesOverrideInSubTheme() {
+    $this->container->get('theme_handler')->install(['test_subtheme']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_subtheme'));
+
+    // Assert that libraries-override specified in the base theme still applies
+    // in sub theme.
+    $library = $library_discovery->getLibraryByName('core', 'drupal.dialog');
+    $this->assertNoAssetInLibrary($library['js'], 'drupal.dialog', 'core/misc/dialog/dialog.js');
+
+    $library = $library_discovery->getLibraryByName('core', 'jquery.farbtastic');
+    $this->assertAssetInLibrary($library['css'], 'jquery.farbtastic', 'core/modules/system/tests/themes/test_basetheme/css/farbtastic.css');
+  }
+
+  /**
    * Asserts that the given asset is in library.
    *
-   * @param mixed[] $library_item
+   * @param mixed $library_item
    *   The library where given asset should be.
    * @param string $library_name
    *   Name of the library.
@@ -156,20 +216,20 @@ public function testLibrariesOverrideMalformedAsset() {
    */
   protected function assertAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
     if (!isset($message)) {
-      $message = SafeMarkup::format('Asset @asset found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+      $message = sprintf('Asset %s found in library %s', $asset, $library_name);
     }
     foreach ($library_item as $definition) {
       if ($asset == $definition['data']) {
-        return($this->pass($message));
+        return $this->pass($message);
       }
     }
-    return($this->fail($message));
+    return $this->fail($message);
   }
 
   /**
    * Asserts that the given asset is not in library.
    *
-   * @param mixed[] $library_item
+   * @param mixed $library_item
    *   The library where given asset should not be.
    * @param string $library_name
    *   Name of the library.
@@ -182,14 +242,14 @@ protected function assertAssetInLibrary($library_item, $library_name, $asset, $m
    */
   protected function assertNoAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
     if (!isset($message)) {
-      $message = SafeMarkup::format('Asset @asset not found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+      $message = sprintf('Asset %s not found in library %s', $asset, $library_name);
     }
     foreach ($library_item as $definition) {
       if ($asset == $definition['data']) {
-        return($this->fail($message));
+        return $this->fail($message);
       }
     }
-    return($this->pass($message));
+    return $this->pass($message);
   }
 
 }
diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
index dcb1a2f..587070c 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
@@ -7,3 +7,6 @@ libraries:
   - test_basetheme/global-styling
 stylesheets-remove:
   - '@theme_test/css/base-remove.css'
+libraries-override:
+  core/drupal.dialog/js/misc/dialog/dialog.js: false
+  core/jquery.farbtastic/css/component/assets/vendor/farbtastic/farbtastic.css: css/farbtastic.css
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 b50181d..32d723c 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
@@ -14,10 +14,9 @@ description: 'Theme for testing the theme system'
 version: VERSION
 base theme: classy
 core: 8.x
--stylesheets-remove:
+stylesheets-remove:
   - '@system/css/system.theme.css'
 libraries-override:
-  system/base/css/component/css/system.module.css: false
   # Replace an entire library.
   core/drupal.collapse: test_theme/collapse
   # Remove an entire library.
@@ -28,6 +27,14 @@ libraries-override:
   core/drupal.dialog/css/theme/misc/dialog.theme.css: false
   # It works for JS as well.
   core/jquery/js/assets/vendor/jquery/jquery.min.js: js/collapse.js
+  # Use Drupal relative paths.
+  core/jquery.once/js/assets/vendor/jquery-once/jquery.once.min.js: /themes/my_theme/js/jquery.once.js
+  # Use streamwrappers.
+  core/drupal.active-link/js/misc/active-link.js: public://my_js/active-link.js
+  # Use protocol-free URI.
+  core/drupal.dialog/js/misc/dialog/dialog.js: //my-server/my_theme/js/dialog.js
+  # Use regular URI.
+  core/jquery.farbtastic/js/component/assets/vendor/farbtastic/farbtastic.js: http://example.com/my_theme/js/farbtastic.js
 regions:
   content: Content
   left: Left