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

tests/compiler/dart2js/erroneous_element_test.dart

Index: tests/compiler/dart2js/erroneous_element_test.dart
diff --git a/tests/compiler/dart2js/erroneous_element_test.dart b/tests/compiler/dart2js/erroneous_element_test.dart
index 3bc0e43cdb185c7f485dd1163858b6fc84ea2003..737f72178a1ae1ff69766cacf93af521060f2eb8 100644
--- a/tests/compiler/dart2js/erroneous_element_test.dart
+++ b/tests/compiler/dart2js/erroneous_element_test.dart
@@ -3,14 +3,13 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:expect/expect.dart';
-import 'package:compiler/src/elements/elements.dart';
 
+import 'package:compiler/src/diagnostics/messages.dart'
+    show MessageKind;
+import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX;
 
-import 'package:compiler/src/messages.dart'
-    show MessageKind;
-
 void main() {
   ErroneousElement e = new ErroneousElementX(MessageKind.GENERIC,
                                              {'text': 'error'},