on() {
      this._updateItems();
    },

    get _shouldUpdateSelection() {
      return this.selected != null;
    },

    _checkFallback: function() {
      if (this._shouldUpdateSelection) {
        this._updateSelected();
      }
    },

    _addListener: function(eventName) {
      this.listen(this, eventName, '_activateHandler');
    },

    _removeListener: function(eventName) {
      this.unlisten(this, eventName, '_activateHandler');
    },

    _activateEventChanged: function(eventName, old) {
  