anchor.diff:    FutureValue()
     : _outputChangedHandle = new EventHandle<EventArgs>(),
     _inputChangedHandle = new EventHandle<EventArgs>(),
  -  _errorHandle = new EventHandle();
  +  _errorHandle = new EventHandle<Object>();
   
   
     TInput get input() => _input;
  
positive.code:     assert(_future == null);
      assert(!_pending);
      _future = getFuture(_input);
      _future.then(_futureCompleted);
    }
  
    TInput _input;
  
negative.code: 
    final EventHandle<EventArgs> _outputChangedHandle;
    final EventHandle<EventArgs> _inputChangedHandle;
    final EventHandle<Object> _errorHandle;
  }
  