Do not export library prefix names used in a library

tests/language/prefix24_lib2.dart

Index: tests/language/prefix24_lib2.dart
===================================================================
--- tests/language/prefix24_lib2.dart	(revision 0)
+++ tests/language/prefix24_lib2.dart	(revision 0)
@@ -0,0 +1,7 @@
+// 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.
+
+library prefix24_lib2;
+
+bar() => "prefix24_lib2_bar";