Fix codegeneration for 'return'.

pkg/compiler/lib/src/js_backend/codegen/codegen.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.
 
 library code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
@@ skipping 264 matching lines @@
   }
 
   @override
   void visitWhileTrue(tree_ir.WhileTrue node) {
     giveup(node);
     // TODO: implement visitWhileTrue
   }
 
   @override
   void visitReturn(tree_ir.Return node) {
-    if (node.value != null) {
-      accumulator.add(new js.Return(visitExpression(node.value)));
-    }
+    accumulator.add(new js.Return(visitExpression(node.value)));
   }
 
   bool isNullLiteral(js.Expression exp) => exp is js.LiteralNull;
 
 }