Added support for exports and re-exports.

tests/language/export_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 export and re-export.
+
+library export_test;
+
+import 'export_helper1.dart';
+import 'export_helper3.dart' as lib;
+
 void main() {
-  var foo = new Foo();
-  var bar = new Bar(); /// 01: compile-time error
+  print(new Exported());
+  print(new ReExported());
+  print(new lib.Exported());
+  print(new lib.ReExported());
 }
 
-class Foo {
-  factory Foo() => null;
-}
-
-class Bar extends Foo {
-  Bar(); /// 01: continued
-}