the focus target
           * for the `iron-dropdown` contained by `paper-menu-button`.
           */
          _dropdownContent: {
            type: Object
          }
        },

        hostAttributes: {
          role: 'group',
          'aria-haspopup': 'true'
        },

        listeners: {
          'iron-activate': '_onIronActivate',
          'iron-select': '_onIronSelect'
        },

        /**
         * The content element that is contained by the menu button, if any.
         */
        get c