anchor.diff:        return new Future.value(_map[key]);
       } else {
         return _storageArea.get(key).then((Map<String, String> map) {
  -        // TODO(ussuri): Shouldn't we cache the just read value in _map?
           final String val = map == null ? null : map[key];
           return val != null ? val : defaultVal;
         });
  
positive.code:   void flush() {
      if (_map.isNotEmpty) {
        _storageArea.set(_map);
        _logger.info('saved preferences: ${_map.keys}');
        _map.clear();
      }
  
  
negative.code:  * A PreferenceStore backed by `chome.storage.local`.
   */
  PreferenceStore localStore = new _ChromePreferenceStore(
      chrome.storage.local, 'local', new Duration(milliseconds: 100));
  
  /**
   * A PreferenceStore backed by `chome.storage.sync`.
   */
  PreferenceStore syncStore = new _ChromePreferenceStore(
      chrome.storage.sync, 'sync', new Duration(milliseconds: 100));
  
  /**
   * Preferences specific to Spark.
  