anchor.diff:    factory HeadingElement.h1() => document.createElement("h1");
     factory HeadingElement.h2() => document.createElement("h2");
     factory HeadingElement.h3() => document.createElement("h3");
  +}
  +
  +class ImageElement extends HtmlEment {
  +  String src;
   }
   
   class InputElement extends HtmlElement {
  
positive.code: }
  
  class MediaElement extends Element {}
  
  dynamic JS(a, b, c, d) {}
  ''',
  
negative.code: abstract class ElementStream<T extends Event> implements Stream<T> {}
  
  abstract class Element {
    factory Element.html(String html,
            {NodeValidator validator, NodeTreeSanitizer treeSanitizer}) =>
        new HtmlElement();
  
    /// Stream of `cut` events handled by this [Element].
    ElementStream<Event> get onCut => throw 0;
  
  