anchor.diff:      request[2] = length;
       _fileService.call(request).then((response) {
         if (_isErrorResponse(response)) {
  -        _handleErrorResponse(response, "truncate failed");
  +        _handleErrorResponse(response, "truncate failed for file '$_name'");
         } else {
           callback();
         }
  
positive.code:     _checkNotClosed();
      var result = _FileUtils.position(_id);
      if (result is OSError) {
        throw new FileIOException("position failed", result);
      }
      return result;
    }
  
negative.code:     _checkNotClosed();
      var result = _FileUtils.length(_id);
      if (result is OSError) {
        throw new FileIOException("length failed for file '$_name'", result);
      }
      return result;
    }
  