Support `void` as generic argument.

pkg/compiler/lib/src/js_backend/backend_helpers.dart

 // Copyright (c) 2015, 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 dart2js.js_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Uris;
 import '../compiler.dart' show Compiler;
 import '../core_types.dart' show CommonElements, ElementEnvironment;
@@ skipping 693 matching lines @@
 
   ClassEntity get asyncStarController =>
       _findAsyncHelperClass("_AsyncStarStreamController");
 
   ConstructorEntity get asyncStarControllerConstructor =>
       _env.lookupConstructor(asyncStarController, "", required: true);
 
   ConstructorEntity get streamIteratorConstructor =>
       _env.lookupConstructor(_findAsyncHelperClass("StreamIterator"), "");
 
-  ClassEntity get VoidRuntimeType => _findHelperClass('VoidRuntimeType');
 
   FunctionEntity get defineProperty => _findHelperFunction('defineProperty');
 
   FunctionEntity get startRootIsolate =>
       _findLibraryMember(isolateHelperLibrary, START_ROOT_ISOLATE);
 
   FunctionEntity get currentIsolate =>
       _findLibraryMember(isolateHelperLibrary, '_currentIsolate');
 
   FunctionEntity get callInIsolate =>
@@ skipping 39 matching lines @@
   /// The class for patch annotations defined in dart:_js_helper.
   ClassEntity get patchAnnotationClass =>
       _patchAnnotationClass ??= _findHelperClass('_Patch');
 
   ClassEntity _nativeAnnotationClass;
 
   /// The class for native annotations defined in dart:_js_helper.
   ClassEntity get nativeAnnotationClass =>
       _nativeAnnotationClass ??= _findHelperClass('Native');
 }