Implement part directive

tests/language/import_combinators_part.dart

Index: tests/language/import_combinators_part.dart
===================================================================
--- tests/language/import_combinators_part.dart	(revision 0)
+++ tests/language/import_combinators_part.dart	(revision 0)
@@ -0,0 +1,9 @@
+// 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.
+
+// This file is part of the test import_combinators_test.dart
+
+lookBehindCurtain() {
+  return show;  // show is an imported identifier.
+}