s TAB key events to track focus changes.
     * Will wrap focus for overlays withBackdrop.
     * @param {!Event} event
     * @protected
     */
    _onCaptureTab: function(event) {
      if (!this.withBackdrop) {
        return;
      }
      this.__ensureFirstLastFocusables();
      // TAB wraps from last to first focusable.
      // Shift + TAB wraps from first to last focusable.
      var shift = event.shiftKey;
      var nodeToCheck = shift ? this.__firstFocusableNode : this.__lastFocusableNode;
     