code: function preferencesInitialize() {
            return this.initializedPromise = defaultPreferences.then(function (defaults) {
              Object.defineProperty(this, 'defaults', {
                value: Object.freeze(defaults),
                writable: false,
                enumerable: true,
                configurable: false
              });
              this.prefs = cloneObj(defaults);
              return this._readFromStorage(defaults);
            }.bind(this)).then(function (prefObj) {
              this.isInitializedPromiseResolved = true;
              if (prefObj) {
                this.prefs = prefObj;
              }
            }.bind(this));
          }
docs: Initialize and fetch the current preference values from storage.
  @return {Promise} A promise that is resolved when the preferences
  have been initialized.