Support checked mode for return type.

sdk/lib/_internal/compiler/implementation/js_backend/backend.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.
 
 part of js_backend;
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
@@ skipping 889 matching lines @@
   }
 
   HType optimisticFieldType(Element element) {
     return fieldTypes.optimisticFieldType(element);
   }
 
   SourceString getCheckedModeHelper(DartType type) {
     Element element = type.element;
     bool nativeCheck =
           emitter.nativeEmitter.requiresNativeIsCheck(element);
-    if (element == compiler.stringClass) {
+    if (type == compiler.types.voidType) {
+      return const SourceString('voidTypeCheck');
+    } else if (element == compiler.stringClass) {
       return const SourceString('stringTypeCheck');
     } else if (element == compiler.doubleClass) {
       return const SourceString('doubleTypeCheck');
     } else if (element == compiler.numClass) {
       return const SourceString('numTypeCheck');
     } else if (element == compiler.boolClass) {
       return const SourceString('boolTypeCheck');
     } else if (element == compiler.functionClass || element.isTypedef()) {
       return const SourceString('functionTypeCheck');
     } else if (element == compiler.intClass) {
@@ skipping 29 matching lines @@
     print("Inferred return types:");
     print("----------------------");
     dumpReturnTypes();
     print("");
     print("Inferred field types:");
     print("------------------------");
     fieldTypes.dump();
     print("");
   }
 }