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

tests/compiler/dart2js/package_root_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.package_root;
 
 import 'dart:async';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/compiler.dart'
        show DiagnosticHandler, Diagnostic, PackagesDiscoveryProvider;
-import 'package:compiler/src/messages.dart'
+import 'package:compiler/src/diagnostics/messages.dart'
        show MessageKind;
 import 'package:package_config/packages.dart';
 
 import 'memory_compiler.dart';
 import 'memory_source_file_helper.dart';
 
 const MEMORY_SOURCE_FILES = const {
   'main.dart': '''
 
 import 'package:foo/foo.dart';
@@ skipping 54 matching lines @@
     await runTest(
         Uri.parse('package:foo/foo.dart'),
         MessageKind.LIBRARY_NOT_FOUND,
         packageConfig: PACKAGE_CONFIG_URI);
     await runTest(
         Uri.parse('package:foo/foo.dart'),
         MessageKind.LIBRARY_NOT_FOUND,
         packagesDiscoveryProvider: noPackagesDiscovery);
   });
 }