Make pkg/dart_messages strong.

pkg/dart_messages/test/dart_messages_test.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:io' as io;
 
-import '../lib/shared_messages.dart';
+import 'package:dart_messages/shared_messages.dart';
 
 void testJsonIsUpdated() {
   var packageRoot = io.Platform.packageRoot;
   if (packageRoot == null || packageRoot == "") {
     throw new UnsupportedError("This test requires a package root.");
   }
   var jsonUri = Uri
       .parse(packageRoot)
       .resolve('dart_messages/generated/shared_messages.json');
   var jsonPath = jsonUri.toFilePath();
@@ skipping 30 matching lines @@
       throw "Specialization doesn't have same id: $id - $genericId";
     }
   }
 }
 
 void main() {
   testJsonIsUpdated();
   testIdsAreUnique();
   testSpecializationsAreOfSameId();
 }