BREAKING CHANGE: enforce part of directive

tests/language/private3.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 PrivateOther;
+
 class LibOther3 {
   static accessFieldA3(var a) => a.fieldA;
   static accessFieldB3(var b) => b._fieldB;
   static int c_field1b(c) => c._field1;
   static int c_field2b(c) => c._field2;
 }
 
 class AImported extends AExposed {
   AImported() : super();
   getFieldA() => fieldA;
 }
 
 class C2 extends C1 {
   int _field1;
   C2() : super(), _field1 = 99;
 
   field1b() => _field1;
 }
 
 class C4 extends C3 {
   int _field2;
   C4() : super(), _field2 = 1024;
 
   field2b() => _field2;
   field1d() => _field1;
 }