- Add test for private dart:_ libraries.

language/import_private_test.dart

Index: language/import_private_test.dart
===================================================================
--- language/import_private_test.dart	(revision 0)
+++ language/import_private_test.dart	(revision 0)
@@ -0,0 +1,10 @@
+// Copyright (c) 2013, 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.
+// Check that private dart:_ libraries cannot be imported.
+
+import "dart:_collection-dev";  /// 01: compile-time error
+
+main() {
+  print("Done.");
+}