anchor.diff:      if (refactoring == null) {
         initStatus =
             new RefactoringStatus.fatal('Unable to create a refactoring');
  -      return;
  +      return null;
       }
       // check initial conditions
       initStatus = await refactoring.checkInitialConditions();
  
positive.code:       feedback.elementKindName = refactoring.elementKindName;
        feedback.oldName = refactoring.oldName;
      }
    }
  
    void _reset([engine.AnalysisContext context]) {
  
negative.code:    * Initializes this context to perform a refactoring with the specified
     * parameters. The existing [Refactoring] is reused or created as needed.
     */
    Future _init(RefactoringKind kind, String file,
        int offset, int length) async {
      await server.onAnalysisComplete;
      // check if we can continue with the existing Refactoring instance
  