Make pkg/dart_messages strong.

pkg/dart_messages/bin/publish.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:convert';
 import 'dart:io' as io;
 
-import '../lib/shared_messages.dart';
+import 'package:dart_messages/shared_messages.dart';
 
 const String jsonPath = '../lib/generated/shared_messages.json';
 const String dart2jsPath =
     '../../compiler/lib/src/diagnostics/generated/shared_messages.dart';
 const String analyzerPath =
     '../../analyzer/lib/src/generated/generated/shared_messages.dart';
 
 final String dontEditWarning = """
 /*
 DON'T EDIT. GENERATED. DON'T EDIT.
@@ skipping 223 matching lines @@
 /// Translates the shared messages in `../lib/shared_messages.dart` to JSON,
 /// dart2js, and analyzer formats.
 ///
 /// Emits the json-output to [jsonPath], the dart2js-output to [dart2jsPath],
 /// and the analyzer-output to [analyzerPath].
 void main() {
   emitJson();
   emitDart2js();
   emitAnalyzer();
 }