anchor.diff:    /// The completer that produced this operation.
     ///
     /// That completer is canceled when [cancel] is called.
  -  final CancelableCompleter<T> _completer;
  +  CancelableCompleter<T> _completer;
   
     CancelableOperation._(this._completer);
   
  
positive.code:     var completer = Completer<T?>.sync();
      value.then(completer.complete, onError: completer.completeError);
  
      _completer._cancelCompleter.future.then((_) {
        completer.complete(cancellationValue);
      }, onError: completer.completeError);
  
  
negative.code:     var controller =
          StreamController<T>(sync: true, onCancel: _completer._cancel);
  
      _completer._inner?.future.then((value) {
        controller.add(value);
        controller.close();
      }, onError: (Object error, StackTrace stackTrace) {
  