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

tests/compiler/dart2js/missing_file_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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.missing_file;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
-import "package:compiler/src/messages.dart";
+import "package:compiler/src/diagnostics/messages.dart";
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 
 const MEMORY_SOURCE_FILES = const {
   'main.dart': '''
 
 import 'foo.dart';
 
 main() {}
 ''',
@@ skipping 17 matching lines @@
 void main() {
   asyncTest(() async {
     await runTest(
         Uri.parse('memory:main.dart'), MessageKind.READ_SCRIPT_ERROR);
     await runTest(
         Uri.parse('memory:foo.dart'), MessageKind.READ_SCRIPT_ERROR);
     await runTest(
         Uri.parse('dart:foo'), MessageKind.READ_SCRIPT_ERROR);
   });
 }