diff -u b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -25,37 +25,19 @@
    *   Array of root paths keyed by the corresponding namespace to look for
    *   plugin implementations, \Plugin\$owner\$type will be appended to each
    *   namespace.
-   * @param array $plugin_namespaces
-   *   An array of paths keyed by it's corresponding namespaces.
    */
-  function __construct($owner, $type, array $root_namespaces = array(), array $plugin_namespaces = array()) {
+  function __construct($owner, $type, array $root_namespaces = array()) {
     $this->owner = $owner;
     $this->type = $type;
-    $this->rootNamespaces = $root_namespaces;
     $annotation_namespaces = array(
       'Drupal\Component\Annotation' => DRUPAL_ROOT . '/core/lib',
       'Drupal\Core\Annotation' => DRUPAL_ROOT . '/core/lib',
     );
-    parent::__construct($plugin_namespaces, $annotation_namespaces, 'Drupal\Core\Annotation\Plugin');
-  }
-
-  /**
-   * Overrides Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery::getPluginNamespaces().
-   *
-   * @todo Figure out how to let this comment still be TRUE.
-   * This is overridden rather than set in the constructor, because Drupal
-   * modules can be enabled (and therefore, namespaces registered) during the
-   * lifetime of a plugin manager.
-   */
-  protected function getPluginNamespaces() {
-    if (!empty($this->pluginNamespaces)) {
-      return parent::getPluginNamespaces();
-    }
     $plugin_namespaces = array();
-    foreach ($this->rootNamespaces as $namespace => $dir) {
+    foreach ($root_namespaces as $namespace => $dir) {
       $plugin_namespaces["$namespace\\Plugin\\{$this->owner}\\{$this->type}"] = array($dir);
     }
-    return $plugin_namespaces;
+    parent::__construct($plugin_namespaces, $annotation_namespaces, 'Drupal\Core\Annotation\Plugin');
   }
 
 }
diff -u b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
@@ -61,16 +61 @@
-  /**
-   * Tests AnnotatedClassDiscovery without root namespaces.
-   */
-  public function testAnnotatedClassDiscoveryWithPluginNamespaces() {
-    $plugin_namespaces = array();
-    $plugin_namespaces['Drupal\\plugin_test\\Plugin\\plugin_test\\fruit'] = array(DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib');
-    $this->discovery = new AnnotatedClassDiscovery('plugin_test', 'fruit', array(), $plugin_namespaces);
-    $plugin_namespaces = array();
-    $plugin_namespaces['Drupal\\plugin_test\\Plugin\\non_existing_module\\non_existing_plugin_type'] = array(DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib');
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('non_existing_module', 'non_existing_plugin_type', array(), $plugin_namespaces);
-
-    $this->testDiscoveryInterface();
-  }
-
 }
-