Use metadata for patching.

tests/utils/dummy_compiler_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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
@@ skipping 82 matching lines @@
 getDispatchProperty(o) {}
 setDispatchProperty(o, v) {}
 var mapTypeToInterceptor;""";
   } else if (uri.path.endsWith('js_helper.dart')) {
     return """
 library jshelper; class JSInvocationMirror {}
 class ConstantMap {} class TypeImpl {}
 createRuntimeType(String name) => null;
 class Closure {}
 class BoundClosure extends Closure {}
+const patch = 0;
 """;
   } else if (uri.path.endsWith('isolate_helper.dart')) {
     return 'library isolatehelper; class _WorkerStub {}';
   } else {
     return "library lib${uri.path.replaceAll('/', '.')};";
   }
 }
 
 Future<String> provider(Uri uri) {
   String source;
@@ skipping 23 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   }).then(asyncSuccess);
 }