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

tests/compiler/dart2js/mock_compiler.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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler.dart' as api;
 import 'package:compiler/src/constants/expressions.dart';
+import 'package:compiler/src/diagnostics/messages.dart';
 import 'package:compiler/src/diagnostics/source_span.dart';
 import 'package:compiler/src/diagnostics/spannable.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/js_backend/js_backend.dart'
     show JavaScriptBackend;
-import 'package:compiler/src/messages.dart';
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/resolution/resolution.dart';
 import 'package:compiler/src/script.dart';
 import 'package:compiler/src/tree/tree.dart';
 import 'package:compiler/src/old_to_new_api.dart';
 import 'parser_helper.dart';
 
 import 'package:compiler/src/elements/modelx.dart'
     show ElementX,
          LibraryElementX,
@@ skipping 375 matching lines @@
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement);
 
   get node => null;
 
   parseNode(_) => null;
 
   bool get hasNode => false;
 }