r
     * should not be notified by the current element. Return true if an element
     * should be notified, or false if it should not be notified.
     *
     * @param {HTMLElement} element A candidate descendant element that
     * implements `IronResizableBehavior`.
     * @return {boolean} True if the `element` should be notified of resize.
     */
    resizerShouldNotify: function(element) { return true; },

    _onDescendantIronResize: function(event) {
      if (this._notifyingDescendant) {
        e