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

tests/compiler/dart2js/unparser2_test.dart

Index: tests/compiler/dart2js/unparser2_test.dart
diff --git a/tests/compiler/dart2js/unparser2_test.dart b/tests/compiler/dart2js/unparser2_test.dart
index ca14149ad2ede626193cb1147a45a8a0b7ebe918..00a30306c5c2f0f97596ce4ea7766f649886d7c6 100644
--- a/tests/compiler/dart2js/unparser2_test.dart
+++ b/tests/compiler/dart2js/unparser2_test.dart
@@ -6,7 +6,7 @@ import "package:expect/expect.dart";
 import "package:compiler/src/scanner/scannerlib.dart";
 import "package:compiler/src/tree/tree.dart";
 
-import "package:compiler/src/diagnostic_listener.dart";
+import "package:compiler/src/diagnostics/diagnostic_listener.dart";
 import "package:compiler/src/elements/elements.dart"
     show CompilationUnitElement,
          LibraryElement;