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

pkg/compiler/lib/src/elements/elements.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 elements;
 
 import '../compiler.dart' show
     Compiler,
     isPrivateName;
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostic_listener.dart';
+import '../diagnostics/diagnostic_listener.dart';
+import '../diagnostics/messages.dart' show MessageKind;
 import '../diagnostics/source_span.dart' show
     SourceSpan;
 import '../diagnostics/spannable.dart' show
     Spannable;
-import '../messages.dart' show MessageKind;
 import '../resolution/resolution.dart';
 import '../ordered_typeset.dart' show OrderedTypeSet;
 import '../scanner/scannerlib.dart' show
     Token,
     isUserDefinableOperator,
     isMinusOperator;
 import '../script.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
 
@@ skipping 1619 matching lines @@
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }