ast focusable nodes if any of them is not set.
     * @private
     */
    __ensureFirstLastFocusables: function() {
      if (!this.__firstFocusableNode || !this.__lastFocusableNode) {
        var focusableNodes = this._focusableNodes;
        this.__firstFocusableNode = focusableNodes[0];
        this.__lastFocusableNode = focusableNodes[focusableNodes.length - 1];
      }
    },

    /**
     * Tasks executed when opened changes: prepare for the opening, move the
     * focus, update the manager, render 