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

tests/compiler/dart2js/library_resolution_test.dart

Index: tests/compiler/dart2js/library_resolution_test.dart
diff --git a/tests/compiler/dart2js/library_resolution_test.dart b/tests/compiler/dart2js/library_resolution_test.dart
index 9aac5b262e87bc056bbdf0d08a4551ba388bcf25..5841727200d888b26917a08f9fe936787c3fad81 100644
--- a/tests/compiler/dart2js/library_resolution_test.dart
+++ b/tests/compiler/dart2js/library_resolution_test.dart
@@ -17,13 +17,13 @@ import "package:async_helper/async_helper.dart";
 import 'package:expect/expect.dart' show
     Expect;
 
-import 'package:compiler/src/elements/elements.dart' show
-    LibraryElement;
-
-import 'package:compiler/src/messages.dart' show
+import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind,
     MessageTemplate;
 
+import 'package:compiler/src/elements/elements.dart' show
+    LibraryElement;
+
 import 'package:compiler/src/null_compiler_output.dart' show
     NullCompilerOutput;