anchor.diff:    Future<int> position() {
       _ensureFileService();
       Completer<int> completer = new Completer<int>();
  -    if (_isClosed) return _completeWithClosedException(completer);
  +    if (closed) return _completeWithClosedException(completer);
       List request = new List(2);
       request[0] = _FileUtils.POSITION_REQUEST;
       request[1] = _id;
  
positive.code:       });
        return completer.future;
      };
      if (_isClosed) return _completeWithClosedException(completer);
      List request = new List(3);
      request[0] = _FileUtils.READ_LIST_REQUEST;
      request[1] = _id;
  
negative.code:                                        [Encoding encoding = Encoding.UTF_8]) {
      _ensureFileService();
      Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
      if (closed) return _completeWithClosedException(completer);
      List request = new List(3);
      request[0] = _FileUtils.WRITE_STRING_REQUEST;
      request[1] = _id;
  