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

pkg/compiler/lib/src/resolution/semantic_visitor.dart

Index: pkg/compiler/lib/src/resolution/semantic_visitor.dart
diff --git a/pkg/compiler/lib/src/resolution/semantic_visitor.dart b/pkg/compiler/lib/src/resolution/semantic_visitor.dart
index a24e54d4cc50eaaeb4f8bf35331c5d0b640749bf..5658bc9de6469fda8bbf1c85c13fdf67ffda3777 100644
--- a/pkg/compiler/lib/src/resolution/semantic_visitor.dart
+++ b/pkg/compiler/lib/src/resolution/semantic_visitor.dart
@@ -8,12 +8,12 @@ import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../diagnostics/invariant.dart' show
     invariant;
+import '../diagnostics/messages.dart' show
+    MessageKind;
 import '../diagnostics/spannable.dart' show
     Spannable,
     SpannableAssertionFailure;
 import '../elements/elements.dart';
-import '../messages.dart' show
-    MessageKind;
 import '../tree/tree.dart';
 import '../universe/universe.dart';