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:       expect(b.models, [aModel]);
        expect(c.models, [cModel]);
        expect(d.models, [cModel]);
  
        a.remove();
        return nextStep;
      });
    });
  }
  