Supported versioned patching.

sdk/lib/_internal/compiler/js_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:_js_embedded_names' show
     GET_TYPE_FROM_NAME,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 57 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();
-
 
 /// Marks the internal map in dart2js, so that internal libraries can is-check
 // them.
 abstract class InternalMap {
 }
 
 /// No-op method that is called to inform the compiler that preambles might
 /// be needed when executing the resulting JS file in a command-line
 /// JS engine.
 requiresPreamble() {}
@@ skipping 3361 matching lines @@
   throw new MainError("No top-level function named 'main'.");
 }
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }