anchor.diff:  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;
   
  
positive.code:   factory HeadingElement.h1() => document.createElement("h1");
    factory HeadingElement.h2() => document.createElement("h2");
    factory HeadingElement.h3() => document.createElement("h3");
  }
  
  class InputElement extends HtmlElement {
  
negative.code:   String src;
  }
  
  class OptionElement extends HtmlElement {
    factory OptionElement({String data: '', String value : '', bool selected: false}) {
    }
  