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

pkg/compiler/lib/src/helpers/helpers.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.
 
 /// Library for debugging helpers. The unittest analyze_unused_test checks that
 /// the helper are not used in production code.
 
 library dart2js.helpers;
 
 import 'dart:async' show
     EventSink;
 import 'dart:collection';
 import 'dart:convert';
 
 import '../../compiler.dart';
 import '../compiler.dart' show
     Compiler;
 import '../diagnostics/invariant.dart' show
     DEBUG_MODE;
+import '../diagnostics/messages.dart' show
+    MessageKind;
 import '../diagnostics/spannable.dart' show
     Spannable;
-import '../messages.dart' show
-    MessageKind;
 import '../util/util.dart';
 
 part 'debug_collection.dart';
 part 'trace.dart';
 part 'expensive_map.dart';
 part 'expensive_set.dart';
 part 'stats.dart';
 part 'track_map.dart';
 
 /// Global flag to enable [debugPrint]. This should always be `true` by default
@@ skipping 109 matching lines @@
       if (printTrace) {
         trace(msg);
       } else {
         debugPrint(msg);
       }
     }
     return true;
   }
   return false;
 }