Supported versioned patching.

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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 for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 169 matching lines @@
   'NoSideEffects': 'class NoSideEffects {}',
   'NoThrows': 'class NoThrows {}',
   'numberOrStringSuperNativeTypeCast':
       'numberOrStringSuperNativeTypeCast(value) {}',
   'numberOrStringSuperNativeTypeCheck':
       'numberOrStringSuperNativeTypeCheck(value) {}',
   'numberOrStringSuperTypeCast': 'numberOrStringSuperTypeCast(value) {}',
   'numberOrStringSuperTypeCheck': 'numberOrStringSuperTypeCheck(value) {}',
   'numTypeCast': 'numTypeCast(value) {}',
   'numTypeCheck': 'numTypeCheck(value) {}',
-  'patch': 'const patch = 0;',
+  '_Patch': 'class _Patch { final tag; const _Patch(this.tag); }',
+  'patch': 'const patch = const _Patch(null);',
+  'patch_new': 'const patch_new = const _Patch("new");',
+  'patch_old': 'const patch_old = const _Patch("old");',
   'propertyTypeCast': 'propertyTypeCast(x) {}',
   'propertyTypeCheck': 'propertyTypeCheck(value, property) {}',
   'requiresPreamble': 'requiresPreamble() {}',
   'RuntimeFunctionType': 'class RuntimeFunctionType {}',
   'RuntimeTypeGeneric': 'class RuntimeTypeGeneric {}',
   'RuntimeTypePlain': 'class RuntimeTypePlain {}',
   'runtimeTypeToString': 'runtimeTypeToString(type, {onTypeVariable(i)}) {}',
   'S': 'S() {}',
   'setRuntimeTypeInfo': 'setRuntimeTypeInfo(a, b) {}',
   'stringSuperNativeTypeCast': 'stringSuperNativeTypeCast(value) {}',
@@ skipping 160 matching lines @@
 const Map<String, String> DEFAULT_ASYNC_LIBRARY = const <String, String>{
   'Future': 'class Future<T> {}',
   'Stream': 'class Stream<T> {}',
 };
 
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };