BREAKING CHANGE: enforce part of directive

tests/language/lazy_static6_src.dart

 // Copyright (c) 2012, 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.
 
 /*
  * A lot of comments to make sure that the lazy initialization code has a
  * position that does not fit into lazy_static6_test.dart file.
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  ******************************************************************************
  */
+part of lazy_static6_test;
 
 final x = (t) => (u) => t + u;
 
 main() {
   Expect.equals(499, x(498)(1));
   Expect.equals(42, x(39)(3));
 }