Added support for exports and re-exports.

tests/language/export_cyclic_test.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.
+
+// Test cyclic export and re-export.
+
+/**
+ * export_cyclic_test re-exports export_cyclic_helper1 which declares B
+ * export_cyclic_helper1 re-exports export_cyclic_helper2 which declares C
+ * export_cyclic_helper2 re-exports export_cyclic_test which declares A
+ * export_cyclic_helper2 re-exports export_cyclic_helper3 which declares D
+ */
+
+library export_cyclic_test;
+
+import 'export_cyclic_helper1.dart';
+export 'export_cyclic_helper1.dart';
+
+class A {}
+
+void main() {
+  print(new A());
+  print(new B());
+  print(new C());
+  print(new D());
+}
+