anchor.diff:  import '../../pkg/unittest/lib/unittest.dart';
   import '../../pkg/unittest/lib/html_config.dart';
   
  +var stepDuration;
   Future get nextStep {
  -  return new Future.delayed(new Duration());
  +  return new Future.delayed(stepDuration);
   }
   
   class ModelTracker {
  
positive.code: 
        c.clearModel();
        expect(d.model, model2);
      });
    });
  
  
negative.code:   void clearModel() {
      element.clearModel();
    }
  
    void remove() {
      element.remove();
    }
  }
  
  main() {
    useHtmlConfiguration();
  
    if (MutationObserver.supported) {
      stepDuration = new Duration();
    } else {
      // Need to step after the tree update notifications, but on IE9 these may
      // get polyfilled to use setTimeout(0). So use a longer timer to try to
      // get a later callback.
      stepDuration = new Duration(milliseconds: 15);
    }
  
    test('basic top down', () {
      var a = new DivElement();
  