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

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index db9f0a7e8fcd4ffb8fe1f161d87c9e249016abf3..ee7936efc2cb7dd0790f2a01bc886377ea0a9ed4 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -11,16 +11,16 @@ import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostic_listener.dart';
+import '../diagnostics/diagnostic_listener.dart';
 import '../diagnostics/invariant.dart' show
     invariant;
+import '../diagnostics/messages.dart';
 import '../diagnostics/source_span.dart' show
     SourceSpan;
 import '../diagnostics/spannable.dart' show
     Spannable,
     SpannableAssertionFailure;
 import '../helpers/helpers.dart';
-import '../messages.dart';
 import '../ordered_typeset.dart' show
     OrderedTypeSet;
 import '../resolution/resolution.dart';