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

tests/compiler/dart2js/private_test.dart

 // Copyright (c) 2012, 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:async';
 
 import 'package:async_helper/async_helper.dart';
+import 'package:compiler/src/diagnostics/messages.dart' show MessageKind;
 import 'package:compiler/src/io/source_file.dart';
-import 'package:compiler/src/messages.dart' show MessageKind;
 
 import 'mock_compiler.dart';
 
 const String PRIVATE_SOURCE_URI = 'src:private';
 const String PRIVATE_SOURCE = '''
 
 var _privateVariable;
 void _privateFunction() {}
 
 class _PrivateClass {
@@ skipping 144 matching lines @@
     analyze('var value = publicClass.publicGetter;'),
     // Call public setter on public class.
     analyze('publicClass.publicSetter = 0;'),
     // Access public method on public class.
     analyze('var value = publicClass.publicMethod;'),
     // Call public method on public class.
     analyze('publicClass.publicMethod();'),
   ], (f) => f()));
 }