urn only `[firstFocusable, lastFocusable];`
     * @type {Array<Node>}
     * @protected
     */
    get _focusableNodes() {
      return Polymer.IronFocusablesHelper.getTabbableNodes(this);
    },

    ready: function() {
      // Used to skip calls to notifyResize and refit while the overlay is animating.
      this.__isAnimating = false;
      // with-backdrop needs tabindex to be set in order to trap the focus.
      // If it is not set, IronOverlayBehavior will set it, and remove it if with-backdrop = 