Add dart_messages package.

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:convert';
+import 'dart:io';
+
+import '../lib/shared_messages.dart';
+
+void testJsonIsUpdated() {
+  var packageRoot = Platform.packageRoot;
+  if (packageRoot == null || packageRoot == "") {
+    throw new UnsupportedError("This test requires a package root.");
+  }
+  var jsonUri =
+      Uri.parse(packageRoot).resolve('dart_messages/shared_messages.json');
+  var jsonPath = jsonUri.toFilePath();
+  var content = new File(jsonPath).readAsStringSync();
+  if (JSON.encode(MESSAGES) != content) {
+    throw "Content is not the same";

[Siggi Cherem (dart-lang), 2015/12/17 00:06:56]

Consider adding here instructions on how to regenerate it. 

You could even combine this and json_converter.dart, and make the test take a
flag `--regenerate-json` :)

[floitsch, 2015/12/17 07:27:17]

I added instructions what to do when the test fails.
I prefer keeping the test and the generator separate. Test is in test/, and
converter is in bin/.

+  }
+}
+
+void testIdsAreUnique() {
+  var usedIds = new Set();
+  for (var entry in MESSAGES.values) {
+    var id = entry['id'];
+    if (!usedIds.add(id)) {
+      throw "Id appears twice: $id";
+    }
+  }
+}
+
+void main() {
+  testJsonIsUpdated();
+  testIdsAreUnique();
+}