anchor.diff:        feedback.elementKindName = refactoring.elementKindName;
         feedback.oldName = refactoring.oldName;
       }
  +    return null;
     }
   
     void _reset([engine.AnalysisContext context]) {
  
positive.code:    * Initializes this context to perform a refactoring with the specified
     * parameters. The existing [Refactoring] is reused or created as needed.
     */
    _init(RefactoringKind kind, String file,
        int offset, int length) async {
      await server.onAnalysisComplete;
      // check if we can continue with the existing Refactoring instance
  
negative.code:       await _init(params.kind, params.file, params.offset, params.length);
        if (initStatus.hasFatalError) {
          feedback = null;
          _sendResultResponse();
          return;
        }
        // set options
        if (_requiresOptions) {
          if (params.options == null) {
            optionsStatus = new RefactoringStatus();
            _sendResultResponse();
            return;
          }
          optionsStatus = _setOptions(params);
          if (_hasFatalError) {
            _sendResultResponse();
            return;
          }
        }
        // done if just validation
        if (params.validateOnly) {
          finalStatus = new RefactoringStatus();
          _sendResultResponse();
          return;
        }
        // simulate an exception
        if (test_simulateRefactoringException_final) {
  