Support `void` as generic argument.

sdk/lib/_internal/js_runtime/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
         DEFERRED_LIBRARY_URIS,
         DEFERRED_LIBRARY_HASHES,
@@ skipping 165 matching lines @@
 // TODO(floitsch): move this to foreign_helper.dart or similar.
 @ForceInline()
 bool isNullTypeRti(type) {
   return JS_BUILTIN(
       'returns:bool;effects:none;depends:none',
       JsBuiltin.isGivenTypeRti,
       type,
       JS_GET_NAME(JsGetName.NULL_CLASS_TYPE_NAME));
 }
 
+/// Returns whether the given type is void type.
+// TODO(floitsch): move this to foreign_helper.dart or similar.
+@ForceInline()
+bool isVoidType(rti) {
+  return JS_BUILTIN(
+      'returns:bool;effects:none;depends:none', JsBuiltin.isVoidRti, rti);
+}
+
 /// Returns the metadata of the given [index].
 // TODO(floitsch): move this to foreign_helper.dart or similar.
 @ForceInline()
 getMetadata(int index) {
   return JS_BUILTIN(
       'returns:var;effects:none;depends:none', JsBuiltin.getMetadata, index);
 }
 
 /// Returns the type of the given [index].
 // TODO(floitsch): move this to foreign_helper.dart or similar.
@@ skipping 3600 matching lines @@
 // unneeded code.
 class _UnreachableError extends AssertionError {
   _UnreachableError();
   String toString() => "Assertion failed: Reached dead code";
 }
 
 @NoInline()
 void assertUnreachable() {
   throw new _UnreachableError();
 }