anchor.diff:        : _isSync = isSync,
           _runInZone = runInZone,
           _onListen = onListen,
  -        _onCancel = onCancel,
  -        super(const Stream<Null>.empty());
  +        _onCancel = onCancel;
   
     bool get isSync => _isSync;
   
  
positive.code:             onCancel: onCancel);
  
    /// Returns `this`.
    Stream<T> get stream => this;
  
    /// Returns `this`.
    EventSink<T> get sink => this;
  }
  
negative.code:   }
  }
  
  /// A [SimpleStream] implementation of [StreamSubscription].
  ///
  /// Major differences:
  /// * [cancel] returns null.
  /// * Using [pause] is not supported.
  ///
  class SimpleStreamSubscription<T> implements StreamSubscription<T> {
    @override
    final bool isPaused = false;
    SimpleStream<T> _stream;
    StreamCallbackFunc<T> _callback;
    Zone _contextZone;
    Function _doneCallback;
    Function _onError;
    bool _cancelOnError = false;
  