Use metadata for patching.

sdk/lib/_internal/compiler/implementation/compiler.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.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 628 matching lines @@
   ClassElement listClass;
   ClassElement typeClass;
   ClassElement mapClass;
   ClassElement symbolClass;
   ClassElement stackTraceClass;
   ClassElement typedDataClass;
 
   /// The constant for the [proxy] variable defined in dart:core.
   Constant proxyConstant;
 
+  /// The constant for the [patch] variable defined in dart:_js_helper.
+  Constant patchConstant;
+
   // Initialized after symbolClass has been resolved.
   FunctionElement symbolConstructor;
 
   // Initialized when dart:mirrors is loaded.
   ClassElement mirrorSystemClass;
 
   // Initialized when dart:mirrors is loaded.
   ClassElement mirrorsUsedClass;
 
   // Initialized after mirrorSystemClass has been resolved.
@@ skipping 407 matching lines @@
   Future onLibrariesLoaded(Map<Uri, LibraryElement> loadedLibraries) {
     return new Future.sync(() {
       if (!loadedLibraries.containsKey(DART_CORE)) return new Future.value();
 
       functionClass.ensureResolved(this);
       functionApplyMethod = functionClass.lookupLocalMember('apply');
 
       proxyConstant =
           resolver.constantCompiler.compileConstant(coreLibrary.find('proxy'));
 
+      patchConstant = resolver.constantCompiler.compileConstant(
+          jsHelperLibrary.find('patch'));
+
       if (jsInvocationMirrorClass != null) {
         jsInvocationMirrorClass.ensureResolved(this);
         invokeOnMethod = jsInvocationMirrorClass.lookupLocalMember(INVOKE_ON);
       }
 
       if (preserveComments) {
         return libraryLoader.loadLibrary(DART_MIRRORS)
             .then((LibraryElement libraryElement) {
           documentClass = libraryElement.find('Comment');
         });
@@ skipping 897 matching lines @@
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }
 
 /// Information about suppressed warnings and hints for a given library.
 class SuppressionInfo {
   int warnings = 0;
   int hints = 0;
 }