Return 'null' rename refactoring in case of a null element.

pkg/analysis_server/test/services/refactoring/rename_constructor_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.services.refactoring.rename_constructor;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/services/correction/status.dart';
+import 'package:analysis_server/src/services/refactoring/refactoring.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../utils.dart';
 import 'abstract_rename.dart';
 
 main() {
   initializeTestEnvironment();
@@ skipping 173 matching lines @@
 class B extends A {
   B() : super() {}
   factory B._() = A;
 }
 main() {
   new A();
 }
 ''');
   }
 
+  void test_newInstance_nullElement() {
+    RenameRefactoring refactoring = new RenameRefactoring(searchEngine, null);
+    expect(refactoring, isNull);
+  }
+
   void _createConstructorDeclarationRefactoring(String search) {
     ConstructorElement element = findNodeElementAtString(
         search, (node) => node is ConstructorDeclaration);
     createRenameRefactoringForElement(element);
   }
 }