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

tests/compiler/dart2js/erroneous_element_test.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.
 
 import 'package:expect/expect.dart';
+
+import 'package:compiler/src/diagnostics/messages.dart'
+    show MessageKind;
 import 'package:compiler/src/elements/elements.dart';
-
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX;
 
-import 'package:compiler/src/messages.dart'
-    show MessageKind;
-
 void main() {
   ErroneousElement e = new ErroneousElementX(MessageKind.GENERIC,
                                              {'text': 'error'},
                                              'foo', null);
   Expect.stringEquals('<foo: error>', '$e');
 }