Fix co19 test LibTest/html/IFrameElement/IFrameElement.created_A01_t01

[terrylucas]

Fix the test co19/LibTest/html/IFrameElement/IFrameElement.created_A01_t01

To create an IFrame custom element do the following and the test will work.

class MyIFrameElement extends IFrameElement {
  factory MyIFrameElement() {
    return new Element.tag('my-iframe');
  }

  MyIFrameElement.created() : super.created();

  void testit() {
    print("In testit 222");
  }
}



iframe_test() {
  var ife;
  var tag = 'my-iframe';
  document.registerElement(tag, MyIFrameElement, extendsTag: 'iframe');

  ife = document.createElement('iframe', 'my-iframe');
  document.body.append(ife);

  ife.testit();
  print(ife);
}

[sgrekhov]

Fixed in both master and strong-mode branches