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

tests/compiler/dart2js/analyze_helper.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 analyze_helper;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:compiler/compiler.dart' as api;
 import 'package:compiler/src/apiimpl.dart';
+import 'package:compiler/src/diagnostics/messages.dart' show
+    Message;
 import 'package:compiler/src/filenames.dart';
-import 'package:compiler/src/messages.dart' show
-    Message;
 import 'package:compiler/src/source_file_provider.dart';
 import 'package:compiler/src/util/uri_extras.dart';
 
 /**
  * Map of whitelisted warnings and errors.
  *
  * Only add a whitelisting together with a bug report to dartbug.com and add
  * the bug issue number as a comment on the whitelisting.
  *
  * Use an identifiable suffix of the file uri as key. Use a fixed substring of
@@ skipping 164 matching lines @@
       exit(1);
     }
   }
   if (analyzeAll) {
     compiler.librariesToAnalyzeWhenRun = uriList;
     return compiler.run(null).then(onCompletion);
   } else {
     return compiler.run(uriList.single).then(onCompletion);
   }
 }