BREAKING CHANGE: enforce part of directive

tests/language/private_main.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.
 // Dart test for testing access to private fields.
 
+part of Private3Test.dart;
+
 main() {
   PrivateMain.main();
 }
 
 class PrivateMain {
 
   static const _myPrecious = "A Ring";
 
   static accessMyPrivates() {
     var value = 0;
@@ skipping 32 matching lines @@
     }
     Expect.equals(-1, value);
   }
 
   static main() {
     accessMyPrivates();
     accessMyLibPrivates();
     accessOtherLibPrivates();
   }
 }