Move diagnostic_listener.dart and messages.dart to the diagnostics folder

tests/compiler/dart2js/message_kind_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
-import 'package:compiler/src/messages.dart' show
+import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind,
     MessageTemplate;
 
 import 'message_kind_helper.dart';
 
 main(List<String> arguments) {
   List<MessageTemplate> examples = <MessageTemplate>[];
   for (MessageKind kind in MessageKind.values) {
     MessageTemplate template = MessageTemplate.TEMPLATES[kind];
     Expect.isNotNull(template, "No template for $kind.");
@@ skipping 18 matching lines @@
   asyncTest(() => Future.forEach(examples, (MessageTemplate template) {
     print("Checking '${template.kind}'.");
     Stopwatch sw = new Stopwatch()..start();
     return check(template, cachedCompiler).then((var compiler) {
       cachedCompiler = compiler;
       sw.stop();
       print("Checked '${template.kind}' in ${sw.elapsedMilliseconds}ms.");
     });
   }));
 }