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

tests/compiler/dart2js/message_kind_helper.dart

Index: tests/compiler/dart2js/message_kind_helper.dart
diff --git a/tests/compiler/dart2js/message_kind_helper.dart b/tests/compiler/dart2js/message_kind_helper.dart
index 510ab07d453f2cae280d55d3d6a5e78ee9c43c7d..6c4df24b93574ac72dfec9079c1d4fefb52a3326 100644
--- a/tests/compiler/dart2js/message_kind_helper.dart
+++ b/tests/compiler/dart2js/message_kind_helper.dart
@@ -9,11 +9,11 @@ import 'dart:async';
 
 import 'package:compiler/src/compiler.dart' show
     Compiler;
-import 'package:compiler/src/messages.dart' show
-    MessageKind,
-    MessageTemplate;
 import 'package:compiler/src/dart_backend/dart_backend.dart' show
     DartBackend;
+import 'package:compiler/src/diagnostics/messages.dart' show
+    MessageKind,
+    MessageTemplate;
 import 'package:compiler/src/old_to_new_api.dart' show
     LegacyCompilerDiagnostics;