Support user generated custom native JS classes.

tests/html/json_helper.dart

Index: tests/html/json_helper.dart
diff --git a/tests/language/deferred_type_dependency_lib3.dart b/tests/html/json_helper.dart
similarity index 69%
copy from tests/language/deferred_type_dependency_lib3.dart
copy to tests/html/json_helper.dart
index 9a23fc960f3a41a78bc94be3ec5b94d01b130256..15f92c0e61499b2b4316e24228783114d1d66c6b 100644
--- a/tests/language/deferred_type_dependency_lib3.dart
+++ b/tests/html/json_helper.dart
@@ -2,9 +2,9 @@
 // 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 lib3;
+@Js("JSON")
+library json_helper;
 
-class A {
-  var p = true;
-}
+import 'package:js/js.dart';
 
+external String stringify(object);