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.
+// An update to this file must be followed by regenerating the corresponding
+// json file. Use `json_converter.dart` in the bin directory.
+//
+// Every message in this file must have an id. Use `message_id.dart` in the
+// bin directory to generate a fresh one.
 
 // The messages in this file should meet the following guide lines:
 //
 // 1. The message should be a complete sentence starting with an uppercase
 // letter, and ending with a period.
 //
 // 2. Reserved words and embedded identifiers should be in single quotes, so
 // prefer double quotes for the complete message. For example, "The
 // class '#{className}' can't use 'super'." Notice that the word 'class' in the
 // preceding message is not quoted as it refers to the concept 'class', not the
@@ skipping 31 matching lines @@
 // 10. Prefer to not use imperative tone. That is, the message should not sound
 // accusing or like it is ordering the user around. The computer should
 // 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 = {
+final Map<String, Map> MESSAGES = {
 };