BREAKING CHANGE: enforce part of directive

tests/language/library_prefixes.dart

 // Copyright (c) 2011, 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.
 
+part of LibraryPrefixes.lib;
+
 class LibraryPrefixes {
 
   static void main(var expectEquals) {
     var a = Constants.PI;
     var b = other.Constants.PI;
     expectEquals(3.14, a);
     expectEquals(3.14, b);
 
     expectEquals(1, Constants.foo);
     expectEquals(2, other.Constants.foo);
@@ skipping 46 matching lines @@
     expectEquals(9, new E().toto(7)());
     expectEquals(16, new other.E().toto(11)());
 
     expectEquals(111, (new E.fun(100).f)());
     expectEquals(1313, (new other.E.fun(1300).f)());
 
     expectEquals(999, E.fooo(900)());
     expectEquals(2048, other.E.fooo(1024)());
   }
 }