Fix codegeneration for 'return'.

tests/compiler/dart2js/js_backend_cps_ir_test.dart

 // Copyright (c) 2014, 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.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Test that the CPS IR code generator is able to compile the provided
 // example programs.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 46 matching lines @@
   const TestEntry(
   """
 foo() { return 42; }
 main() { return foo(); }
   """,
   """function() {
   return V.foo();
 }"""),
   const TestEntry("main() {}"),
   const TestEntry("main() { return 42; }"),
+  const TestEntry("main() { return; }", """
+function() {
+  return null;
+}"""),
 ];
 
 String formatTest(Map test) {
   return test[TEST_MAIN_FILE];
 }
 
 String getCodeForMain(Compiler compiler) {
   Element mainFunction = compiler.mainFunction;
   js.Node ast = compiler.enqueuer.codegen.generatedCode[mainFunction];
   return js.prettyPrint(ast, compiler).getText();
@@ skipping 14 matching lines @@
           Expect.equals(test.expectation, getCodeForMain(compiler));
         }
       }).catchError((e) {
         print(e);
         Expect.fail('The following test failed to compile:\n'
                     '${formatTest(files)}');
       });
     });
   }
 }