Use metadata for patching.

sdk/lib/_internal/compiler/implementation/compiler.dart

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index 03ff1305f2d5d77b54e410451bb36ea4dde2479d..9a780de48deeb433d90c12930d4f38ea6e7d2146 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -646,6 +646,9 @@ abstract class Compiler implements DiagnosticListener {
   /// 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;
 
@@ -1073,6 +1076,9 @@ abstract class Compiler implements DiagnosticListener {
       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);