Do not export library prefix names used in a library

tests/language/prefix24_lib1.dart

Index: tests/language/prefix24_lib1.dart
===================================================================
--- tests/language/prefix24_lib1.dart	(revision 0)
+++ tests/language/prefix24_lib1.dart	(revision 0)
@@ -0,0 +1,12 @@
+
+// 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_lib1;
+
+import "prefix24_lib2.dart" as X;
+
+// lib1_foo() returns value of bar() in library prefix24_lib2.
+lib1_foo() => X.bar();
+