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

tests/compiler/dart2js/analyze_only_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.
 
 // Smoke test of the dart2js compiler API.
 library analyze_only;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 
 import '../../utils/dummy_compiler_test.dart' as dummy;
 import 'package:compiler/compiler.dart';
 
-import 'package:compiler/src/messages.dart' show
+import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind, MessageTemplate;
 
 import 'output_collector.dart';
 
 runCompiler(String main, List<String> options,
             onValue(String code, List errors, List warnings)) {
   List errors = new List();
   List warnings = new List();
 
   Future<String> localProvider(Uri uri) {
@@ skipping 151 matching lines @@
   // --analyze-signatures-only implies --analyze-only
   runCompiler(
     "",
     ['--analyze-signatures-only', '--analyze-all'],
     (String code, List errors, List warnings) {
       Expect.isNull(code);
       Expect.isTrue(errors.isEmpty);
       Expect.isTrue(warnings.isEmpty);
     });
 }