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

tests/compiler/dart2js/import_mirrors_test.dart

 // Copyright (c) 2014, 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.
 
 // Test that the compiler emits a warning on import of 'dart:mirrors' unless
 // the flag --enable-experimental-mirrors is used.
 
 library dart2js.test.import;
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/src/messages.dart' show MessageKind, MessageTemplate;
+import 'package:compiler/src/diagnostics/messages.dart' show
+    MessageKind,
+    MessageTemplate;
 import 'memory_compiler.dart';
 
 const DIRECT_IMPORT = const {
   '/main.dart': '''
 import 'dart:mirrors';
 
 main() {}
 ''',
 
   'paths':
@@ skipping 382 matching lines @@
       DUAL_INDIRECT_IMPORT3,
       DUAL_INDIRECT_PACKAGE_IMPORT1,
       DIRECT_EXPORT,
       INDIRECT_EXPORT1,
       INDIRECT_EXPORT2,
       INDIRECT_PACKAGE_EXPORT1,
       INDIRECT_PACKAGE_EXPORT2],
       (map) => checkPaths(map)
   ));
 }