diff -u b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
--- b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
@@ -153,7 +153,7 @@
    * Determines whether a given theme is enabled.
    *
    * @param string $theme
-   *   The name of the theme (without the .module extension).
+   *   The name of the theme (without the .theme extension).
    *
    * @return bool
    *   TRUE if the theme is both installed and enabled.
diff -u b/core/modules/breakpoint/src/BreakpointManager.php b/core/modules/breakpoint/src/BreakpointManager.php
--- b/core/modules/breakpoint/src/BreakpointManager.php
+++ b/core/modules/breakpoint/src/BreakpointManager.php
@@ -202,7 +202,7 @@
       $groups = array();
       foreach ($this->getDefinitions() as $plugin_definition) {
         if (!isset($groups[$plugin_definition['group']])) {
-          $groups[] = $plugin_definition['group'];
+          $groups[$plugin_definition['group']] = $plugin_definition['group'];
         }
       }
       $this->cacheBackend->set($this->cacheKey . '::groups', $groups, Cache::PERMANENT, array('breakpoints' => TRUE));
diff -u b/core/modules/breakpoint/tests/src/BreakpointTest.php b/core/modules/breakpoint/tests/src/BreakpointTest.php
--- b/core/modules/breakpoint/tests/src/BreakpointTest.php
+++ b/core/modules/breakpoint/tests/src/BreakpointTest.php
@@ -2,7 +2,7 @@
  
 /**
  * @file
- * Contains \Drupal\breakpoint\Tests\BreakpointTest
+ * Contains \Drupal\breakpoint\Tests\BreakpointTest.
  */
  
 namespace Drupal\breakpoint\Tests;
@@ -53,7 +53,7 @@
   }
  
   /**
-   * Setups the local task default.
+   * Sets up the breakpoint defaults.
    */
   protected function setupBreakpoint() {
     $this->breakpoint = new Breakpoint(array(), $this->pluginId, $this->pluginDefinition);