Use metadata for patching.

sdk/lib/_internal/lib/js_helper.dart

 // Copyright (c) 2013, 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 _js_helper;
 
 import 'dart:collection';
 import 'dart:_isolate_helper' show
     IsolateNatives,
     leaveJsAsync,
@@ skipping 40 matching lines @@
     unmangleGlobalNameIfPreservedAnyways,
     unmangleAllIdentifiersIfPreservedAnyways;
 
 part 'annotations.dart';
 part 'constant_map.dart';
 part 'native_helper.dart';
 part 'regexp_helper.dart';
 part 'string_helper.dart';
 part 'js_rti.dart';
 
+class _Patch {
+  const _Patch();
+}
+
+const _Patch patch = const _Patch();
+
 bool isJsIndexable(var object, var record) {
   if (record != null) {
     var result = dispatchRecordIndexability(record);
     if (result != null) return result;
   }
   return object is JavaScriptIndexingBehavior;
 }
 
 String S(value) {
   if (value is String) return value;
@@ skipping 3241 matching lines @@
     JS('', '#.addEventListener("error", #, false)',
        script, convertDartClosureToJS((event) {
       completer.completeError(
           new DeferredLoadException("Loading $uri failed."));
     }, 1));
     JS('', 'document.body.appendChild(#)', script);
 
     return completer.future;
   });
 }