implement exports, fixes #141

test/codegen/language/export_cyclic_helper1.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 dart.core;
+library export_cyclic_helper1;
 
-/**
- * Runtime representation of a type.
- */
-abstract class Type {}
+import 'export_cyclic_helper2.dart';
+export 'export_cyclic_helper2.dart';
+
+class B {
+  A a;
+  B b;
+  C c;
+  D d;
+}