 set up for transitions.
         *
         * We are careful to reset our explicit heights after a transition
         * completes, so that the property doesn't clip values if the user resizes
         * their viewport.
         */
        _onTransitionEnd: function(event) {
          if (event.path[0] !== this.$.transitionMask) return;
          this.$.transitionMask.style.height = '';
        },

        _descriptorChanged: function() {
          this.toggleAttribute('private',       this.descriptor.priv