Use metadata for patching.

tests/compiler/dart2js/dart_backend_test.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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
@@ skipping 58 matching lines @@
   String get userAgent;
 }
 ''';
 
 const helperLib = r'''
 library js_helper;
 class JSInvocationMirror {}
 assertHelper(a) {}
 class Closure {}
 class BoundClosure {}
+const patch = 0;
 ''';
 
 const foreignLib = r'''
 var JS;
 ''';
 
 const isolateHelperLib = r'''
 class _WorkerStub {
 }
 ''';
@@ skipping 661 matching lines @@
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }