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

tests/compiler/dart2js/type_checker_test.dart

Index: tests/compiler/dart2js/type_checker_test.dart
diff --git a/tests/compiler/dart2js/type_checker_test.dart b/tests/compiler/dart2js/type_checker_test.dart
index 46ae8a25021affe6f76b407d2f11caeca0355d9f..7d9573191ee2f6ee6806622422133c0917f805c1 100644
--- a/tests/compiler/dart2js/type_checker_test.dart
+++ b/tests/compiler/dart2js/type_checker_test.dart
@@ -8,13 +8,13 @@ import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'package:compiler/src/dart_types.dart';
+import 'package:compiler/src/diagnostics/messages.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/modelx.dart' show
     ClassElementX,
     CompilationUnitElementX,
     ElementX,
     FunctionElementX;
-import 'package:compiler/src/messages.dart';
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/resolution/resolution.dart' show
     TreeElements,