BREAKING CHANGE: enforce part of directive

tests/language/private2_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 across class hierarchies.
 
+part of Private2Test;
+
 class A {
   var _f;
   var g;
   A() : _f = 42, g = 43;
 }
 
 class C extends B {
   C() : super();
 }
 
 main() {
   var a = new A();
   print(a.g);
   print(a._f);
   var o = new C();
   print(o.g);  // Access to public field in A.
   print(o._f);  // Access to private field in A is allowed.
 }