Don't emit /* new backend */ comment by default.

tests/compiler/dart2js/backend_dart/dart_new_backend_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.
 
 import '../compiler_helper.dart';
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/implementation/dart2jslib.dart';
 import 'package:compiler/implementation/dart_backend/dart_backend.dart';
+import 'package:compiler/implementation/dart_backend/backend_ast_to_frontend_ast.dart'
+    show INSERT_NEW_BACKEND_COMMENT;
 
 const String TestStaticField = """
 class Foo {
   static int x = 1;
 }
 main() {
   print(Foo.x);
 }
 """;
 
@@ skipping 24 matching lines @@
 Future test(String code, String feature) {
   return compile(code).then((output) {
     if (output == null || !output.contains('main() /* new backend */')) {
       throw 'New backend appears to be deactivated for $feature.\n'
             'Output was: $output';
     }
   });
 }
 
 main() {
+  INSERT_NEW_BACKEND_COMMENT = true;
+
   asyncTest(() => Future.forEach([
     () => test(TestStaticField, 'static fields'),
   ], (f) => f()));
 }