ontentElement() {
          return Polymer.dom(this.$.content).getDistributedNodes()[0];
        },

        /**
         * Toggles the drowpdown content between opened and closed.
         */
        toggle: function() {
          if (this.opened) {
            this.close();
          } else {
            this.open();
          }
        },

        /**
         * Make the dropdown content appear as an overlay positioned relative
         * to the dropdown trigger.
         */
        open: function() {
  