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

tests/compiler/dart2js/js_spec_string_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.
 
 // Unit test of the [NativeBehavior.processSpecString] method.
 
 import 'package:expect/expect.dart';
 import 'package:compiler/src/native/native.dart';
-import 'package:compiler/src/diagnostic_listener.dart';
+import 'package:compiler/src/diagnostics/diagnostic_listener.dart';
 import 'package:compiler/src/universe/universe.dart'
     show SideEffects;
 
 const OBJECT = 'Object';
 const NULL = 'Null';
 
 class Listener implements DiagnosticListener {
   String errorMessage;
   internalError(spannable, message) {
     errorMessage = message;
@@ skipping 255 matching lines @@
   test('gvn:false', expectedGvn: false);
   test('returns:A;gvn:true', returns: ['A'], expectedGvn: true);
   test(' gvn : true ;  returns:A;', returns: ['A'], expectedGvn: true);
   test('gvn:true;returns:A;gvn:true', expectError: true);
 
   test('gvn: true; new: true', expectError: true);
   test('gvn: true; new: false', expectedGvn: true, expectedNew: false);
   test('gvn: false; new: true', expectedGvn: false, expectedNew: true);
   test('gvn: false; new: false', expectedGvn: false, expectedNew: false);
 }