anchor.diff:  
     void close(void callback()) {
       if (_id == 0) {
  -      _reportError(new FileIOException("Cannot close file: $_name"));
  +      _reportError(new FileIOException("Cannot close file '$_name'"));
         return;
       }
       _ensureFileService();
  
positive.code:     request[1] = _id;
      _fileService.call(request).then((response) {
        if (_isErrorResponse(response)) {
          _handleErrorResponse(response, "readByte failed");
        } else {
          callback(response);
        }
  
negative.code:     }
      var result = lengthFromName(name);
      if (result is OSError) {
        throw new FileIOException(
            "Cannot retrieve length of file '$name'", result);
      }
      return result;
    }
  