Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/lib/Drupal/Component/PhpStorage/FileStorage.php

@@ -224,7 +224,7 @@ EOF;
    * @param string $path
    *   A string containing either a file or directory path.
    *
-   * @return boolean
+   * @return bool
    *   TRUE for success or if path does not exist, FALSE in the event of an
    *   error.
    */

core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php

@@ -58,7 +58,8 @@ class MTimeProtectedFileStorage extends MTimeProtectedFastFileStorage {
    *
    * @param string $name
    *   The virtual file name. Can be a relative path.
-   * return string
+   *
+   * @return string|false
    *   The full path where the file is if it is valid, FALSE otherwise.
    */
   protected function checkFile($name) {