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

pkg/compiler/lib/src/dart_backend/dart_backend.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.
 
 library dart_backend;
 
 import 'dart:async' show Future;
 import 'dart:math' show max;
 
 import '../../compiler.dart' show
     CompilerOutputProvider;
 import '../common/backend_api.dart' show
     Backend;
 import '../common/codegen.dart' show
     CodegenWorkItem;
 import '../common/registry.dart' show
     Registry;
 import '../common/resolution.dart' show
     ResolutionCallbacks;
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler,
     isPrivateName;
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
-import '../diagnostic_listener.dart';
+import '../diagnostics/diagnostic_listener.dart';
 import '../diagnostics/invariant.dart' show
     invariant;
+import '../diagnostics/messages.dart' show
+    MessageKind;
 import '../diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE,
     Spannable,
     SpannableAssertionFailure;
 import '../elements/elements.dart';
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer,
     WorldImpact;
 import '../library_loader.dart' show
     LoadedLibraries;
-import '../messages.dart' show
-    MessageKind;
 import '../mirror_renamer/mirror_renamer.dart';
 import '../resolution/resolution.dart' show
     TreeElements,
     TreeElementMapping;
 import '../scanner/scannerlib.dart' show
     StringToken,
     Keyword,
     OPEN_PAREN_INFO,
     CLOSE_PAREN_INFO,
     SEMICOLON_INFO,
     IDENTIFIER_INFO;
 import '../tree/tree.dart';
 import '../universe/universe.dart' show
     Selector,
     UniverseSelector;
 import '../util/util.dart';
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;
 
 part 'backend.dart';
 part 'renamer.dart';
 part 'placeholder_collector.dart';
 part 'outputter.dart';