code: function() {
              var e = document.createElement("button");
              e.style.height = "1px";
              e.style.width = "1px";
              e.style.position = "absolute";
              e.style.left = "-10000em";
              e.style.opacity = 0;
              e.tabIndex = -1;
              this.innerElement.appendChild(e);
              this._modalFocus = e;
          }
docs: Creates a hidden focusable element, used to focus on,
  to enforce modality for browsers in which focus cannot
  be applied to the container box.
  
  @method _createHiddenFocusElement
  @private