Files.app: Create unit tests for MetadataCache.

chrome/test/data/file_manager/unit_tests/mocks/mock_file_entry.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Mock class for FileEntry.
  *
  * @param {string} volumeId Id of the volume containing the entry.
  * @param {string} fullPath Full path for the entry.
  * @constructor
  */
 function MockFileEntry(volumeId, fullPath) {
   this.volumeId = volumeId;
   this.fullPath = fullPath;
 }
+
+/**
+ * Returns fake URL.
+ *
+ * @return {string} Fake URL.
+ */
+MockFileEntry.prototype.toURL = function() {
+  return 'filesystem:' + this.volumeId + this.fullPath;
+};