Support checked mode for return type.

sdk/lib/_internal/compiler/implementation/lib/js_helper.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('dart:_js_helper');
 
 #import('dart:coreimpl');
 #import('dart:collection');
 
 #source('constant_map.dart');
@@ skipping 1369 matching lines @@
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeError(value, property);
 }
 
 listSuperNativeTypeCast(value, property) {
   if (value is List || value == null) return value;
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeCastError(value, property);
 }
 
+voidTypeCheck(value) {
+  if (value == null) return value;
+  throw new TypeErrorImplementation(value, 'void');
+}
+
 /**
  * Special interface recognized by the compiler and implemented by DOM
  * objects that support integer indexing. This interface is not
  * visible to anyone, and is only injected into special libraries.
  */
 abstract class JavaScriptIndexingBehavior {
 }
 
 // TODO(lrn): These exceptions should be implemented in core.
 // When they are, remove the 'Implementation' here.
@@ skipping 72 matching lines @@
     result = new TypeImpl(key);
     JS('var', r'#[#] = #', runtimeTypeCache, key, result);
   }
   return result;
 }
 
 String getRuntimeTypeString(var object) {
   var typeInfo = JS('Object', r'#.builtin$typeInfo', object);
   return JS('String', r'#.runtimeType', typeInfo);
 }