Use metadata for patching.

tests/compiler/dart2js/mock_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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 20 matching lines @@
 
 class WarningMessage {
   Spannable node;
   Message message;
   WarningMessage(this.node, this.message);
 
   toString() => message.toString();
 }
 
 const String DEFAULT_HELPERLIB = r'''
+  const patch = 0;
   wrapException(x) { return x; }
   iae(x) { throw x; } ioore(x) { throw x; }
   guard$array(x) { return x; }
   guard$num(x) { return x; }
   guard$string(x) { return x; }
   guard$stringOrArray(x) { return x; }
   makeLiteralMap(List keyValuePairs) {}
   setRuntimeTypeInfo(a, b) {}
   getRuntimeTypeInfo(a) {}
   stringTypeCheck(x) {}
@@ skipping 542 matching lines @@
 
 class MockElement extends FunctionElementX {
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement, false);
 
   get node => null;
 
   parseNode(_) => null;
 }