Added support for exports and re-exports.

tests/language/export_helper1.dart

Index: tests/language/export_helper1.dart
diff --git a/runtime/bin/socket_macos.h b/tests/language/export_helper1.dart
similarity index 66%
copy from runtime/bin/socket_macos.h
copy to tests/language/export_helper1.dart
index b77857c87d93af32b8e83ffc6a735d5c23cd7689..3a934b96685a0c8bbfc918e5b583d3f28ce0cb0a 100644
--- a/runtime/bin/socket_macos.h
+++ b/tests/language/export_helper1.dart
@@ -2,11 +2,11 @@
 // 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.
 
-#ifndef BIN_SOCKET_MACOS_H_
-#define BIN_SOCKET_MACOS_H_
+library export_helper1;
 
-#include <arpa/inet.h>
-#include <netdb.h>
-#include <sys/socket.h>
+import 'export_helper2.dart';
+export 'export_helper2.dart';
 
-#endif  // BIN_SOCKET_MACOS_H_
+class Exported extends ReExported {
+
+}