code: function(opt_returnValue) {
        if (!this.dialog_.hasAttribute('open')) {
          throw new Error('Failed to execute \'close\' on dialog: The element does not have an \'open\' attribute, and therefore cannot be closed.');
        }
        this.setOpen(false);
  
        // Leave returnValue untouched in case it was set directly on the element
        if (opt_returnValue !== undefined) {
          this.dialog_.returnValue = opt_returnValue;
        }
  
        // Triggering "close" event for any attached listeners on the <dialog>.
        var closeEvent = new supportCustomEvent('close', {
          bubbles: false,
          cancelable: false
        });
        this.dialog_.dispatchEvent(closeEvent);
      }
docs: Closes this HTMLDialogElement. This is optional vs clearing the open
  attribute, however this fires a 'close' event.
  
  @param {string=} opt_returnValue to use as the returnValue