Add dart_messages package.

pkg/dart_messages/bin/json_converter.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';
+
+import '../lib/shared_messages.dart' as shared_messages;
+
+/// Translates the shared messages in `../lib/shared_messages` to JSON and
+/// emits it into `../lib/shared_messages.json`.
+void main() {
+  var input = shared_messages.MESSAGES;
+  var outPath =
+      Platform.script.resolve('../lib/shared_messages.json').toFilePath();
+  print("Input: ${input.length} entries");
+  print("Output: $outPath");
+  new File(outPath).writeAsStringSync(JSON.encode(shared_messages.MESSAGES));
+  print("Done");
+}