Do not export library prefix names used in a library

tests/language/prefix24_test.dart

+// 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_test;
+
+import "package:expect/expect.dart";
+
+// Import a library that contains library prefix X.
+import "prefix24_lib1.dart";
+
+// Import a library that declares class X.
+import "prefix24_lib3.dart";
+
+// Check that the library prefix X that is used in library prefix24_lib1
+// remains private to that library and does not collide with class X
+// defined in (and imported from) prefix24_lib3;
+
+main() {
+  Expect.equals("static method bar of class X", X.bar());
+  Expect.equals("prefix24_lib2_bar", lib1_foo());
+}