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

tests/compiler/dart2js/duplicate_library_test.dart

 // Copyright (c) 2014, 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.
 
 // Test that duplicate library names result in different messages depending
 // on whether the libraries are based on the same resource.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
-import 'package:compiler/src/messages.dart' show MessageKind;
+import 'package:compiler/src/diagnostics/messages.dart' show MessageKind;
 import 'memory_compiler.dart';
 
 void check(String kind,
            Iterable<DiagnosticMessage> messages,
            List<MessageKind> expectedMessageKinds) {
   Expect.equals(messages.length, expectedMessageKinds.length,
       "Unexpected $kind count: $messages");
   int i = 0;
   messages.forEach((DiagnosticMessage message) {
     Expect.equals(expectedMessageKinds[i++], message.message.kind);
@@ skipping 112 matching lines @@
     'foo.dart': """
 library lib;
 """,
     'bar.dart': """
 library lib;
 """},
     warnings: [MessageKind.DUPLICATED_LIBRARY_NAME,
                MessageKind.DUPLICATED_LIBRARY_NAME]);
 }