Cleanup warnings in unit tests.

tests/compiler/dart2js/mirrors_helper.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This file is read by 'mirrors_test.dart'.
  */
 
 library mirrors_helper;
 
@@ skipping 50 matching lines @@
     _field2 = value;
   }
 }
 
 class _PrivateClass {
   var _privateField;
   get _privateGetter => _privateField;
   void set _privateSetter(value) => _privateField = value;
   void _privateMethod() {}
   _PrivateClass._privateConstructor();
-  factory _PrivateClass._privateFactoryConstructor() => new _PrivateClass();
+  factory _PrivateClass._privateFactoryConstructor() => null;
 }
 
 const metadata = const Metadata(null);
 
 class Metadata {
   final data;
   const Metadata(this.data);
 }