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

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 5c165b0e2225b2f35e6ccec4ae550d80fb744cbb..96f4f4a37ace4bc678c1425b31d9730a07402a98 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -16,6 +16,8 @@ import 'constants/values.dart' show
     DeferredConstantValue,
     StringConstantValue;
 import 'dart_types.dart';
+import 'diagnostics/messages.dart' show
+    MessageKind;
 import 'elements/elements.dart' show
     AccessorElement,
     AstElement,
@@ -32,8 +34,6 @@ import 'elements/elements.dart' show
     VoidElement;
 import 'js_backend/js_backend.dart' show
     JavaScriptBackend;
-import 'messages.dart' show
-    MessageKind;
 import 'resolution/resolution.dart' show
     AnalyzableElementX,
     TreeElements;