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