 dropdown opens.
         *
         * @event paper-dropdown-open
         */

        /**
         * Fired when the dropdown closes.
         *
         * @event paper-dropdown-close
         */

        behaviors: [
          Polymer.IronA11yKeysBehavior,
          Polymer.IronControlState
        ],

        properties: {
          /**
           * True if the content is currently displayed.
           */
          opened: {
            type: Boolean,
            value: false,
            notify: true,
 