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

pkg/compiler/lib/src/elements/modelx.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.
 
 library elements.modelx;
 
 import '../compiler.dart' show
     Compiler,
     isPrivateName;
 import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostic_listener.dart';
+import '../diagnostics/diagnostic_listener.dart';
 import '../diagnostics/invariant.dart' show
     invariant;
+import '../diagnostics/messages.dart';
 import '../diagnostics/source_span.dart' show
     SourceSpan;
 import '../diagnostics/spannable.dart' show
     Spannable,
     SpannableAssertionFailure;
 import '../helpers/helpers.dart';
-import '../messages.dart';
 import '../ordered_typeset.dart' show
     OrderedTypeSet;
 import '../resolution/resolution.dart';
 import '../resolution/class_members.dart' show
     ClassMemberMixin;
 import '../scanner/scannerlib.dart' show
     EOF_TOKEN,
     ErrorToken,
     Token;
 import '../script.dart';
@@ skipping 2863 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }