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

pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
index 12038c7310d9dcd2755a56a29d8d3a0c6aae7e8a..e95f7721b77c79e1323b20d78231b05499e50f66 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
@@ -10,6 +10,9 @@ import '../../common.dart';
 
 import '../../constants/values.dart' show ConstantValue, FunctionConstantValue;
 import '../../compiler.dart' show Compiler;
+import '../../diagnostics/messages.dart' show
+    MessageKind;
+
 import '../../elements/elements.dart' show ClassElement, FunctionElement;
 import '../../hash/sha1.dart' show Hasher;
 
@@ -28,9 +31,6 @@ import '../../js_backend/js_backend.dart' show
     Namer,
     ConstantEmitter;
 
-import '../../messages.dart' show
-    MessageKind;
-
 import '../../diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE;