ow.cancelAnimationFrame(this.__raf);
      }
      var self = this;
      this.__raf = window.requestAnimationFrame(function nextAnimationFrame() {
        self.__raf = null;
        callback.call(self);
      });
    }

  };

  /**
  Use `Polymer.IronOverlayBehavior` to implement an element that can be hidden or shown, and displays
  on top of other content. It includes an optional backdrop, and can be used to implement a variety
  of UI controls including dialogs and drop downs. Multiple overlays may be d