Add dart_messages package.

pkg/dart_messages/lib/shared_messages.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.
 
 // This file is parsed by JavaScript and must not use fancy Dart constructs.
 // It can contain JSON like constructs and "//" comments (but not "/*" "*/").
 // It must have one assignment (`final MESSAGES =`).
 // All strings must be raw strings.
 
 // The messages in this file should meet the following guide lines:
@@ skipping 41 matching lines @@
 // describe the problem, not criticize for violating the specification.
 //
 // Other things to keep in mind:
 //
 // Generally, we want to provide messages that consists of three sentences:
 // 1. what is wrong, 2. why is it wrong, 3. how do I fix it. However, we
 // combine the first two in [template] and the last in [howToFix].
 
 final MESSAGES = {
 };