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

pkg/compiler/lib/src/native/native.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 native;
 
 import 'dart:collection' show Queue;
 
 import '../common/registry.dart' show
     Registry;
 import '../compiler.dart' show
     Compiler;
 import '../constants/values.dart';
 import '../dart_types.dart';
-import '../diagnostic_listener.dart';
+import '../diagnostics/diagnostic_listener.dart';
+import '../diagnostics/messages.dart' show MessageKind;
 import '../diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE,
     Spannable;
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer;
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
     BaseClassElementX,
     ElementX,
     FunctionElementX,
     LibraryElementX;
 import '../js/js.dart' as js;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show CodeEmitterTask, NativeEmitter;
-import '../messages.dart' show MessageKind;
 import '../resolution/resolution.dart' show ResolverVisitor;
 import '../scanner/scannerlib.dart';
 import '../ssa/ssa.dart';
 import '../tree/tree.dart';
 import '../universe/universe.dart' show SideEffects;
 import '../util/util.dart';
 
 part 'behavior.dart';
 part 'enqueue.dart';
 part 'js.dart';
@@ skipping 27 matching lines @@
   String quotedName = cls.nativeTagInfo;
   return quotedName.substring(1, quotedName.length - 1).split(',');
 }
 
 List<String> nativeTagsOfClass(ClassElement cls) {
   return nativeTagsOfClassRaw(cls).where((s) => !s.startsWith('!')).toList();
 }
 
 bool nativeTagsForcedNonLeaf(ClassElement cls) =>
     nativeTagsOfClassRaw(cls).contains('!nonleaf');