code: function(event, cb) {
              if (this._cfg.waiAria) {
                  var field = this.getTextInputField();
                  var listWidget = this.controller.getListWidget();
                  var ariaActiveDescendant = listWidget ? listWidget.getSelectedOptionDomId() : null;
                  if (ariaActiveDescendant != null) {
                      field.setAttribute("aria-activedescendant", ariaActiveDescendant);
                  } else {
                      field.removeAttribute("aria-activedescendant");
                  }
              }
              if (cb) {
                  this.$callback(cb, event);
              }
          }
docs: Updates the aria-activedescendant attribute on the input DOM element.
  This method supposes that the popup is open.
  This method is registered as the onchange callback for the list widget,
  if accessibility is enabled. It is also called from _afterDropdownOpen.