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

tests/compiler/dart2js/message_kind_helper.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.test.message_kind_helper;
 
 import 'package:expect/expect.dart';
 import 'dart:async';
 
 import 'package:compiler/src/compiler.dart' show
     Compiler;
-import 'package:compiler/src/messages.dart' show
+import 'package:compiler/src/dart_backend/dart_backend.dart' show
+    DartBackend;
+import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind,
     MessageTemplate;
-import 'package:compiler/src/dart_backend/dart_backend.dart' show
-    DartBackend;
 import 'package:compiler/src/old_to_new_api.dart' show
     LegacyCompilerDiagnostics;
 
 import 'memory_compiler.dart';
 
 const String ESCAPE_REGEXP = r'[[\]{}()*+?.\\^$|]';
 
 /// Most examples generate a single diagnostic.
 /// Add an exception here if a single diagnostic cannot be produced.
 /// However, consider that a single concise diagnostic is easier to understand,
@@ skipping 115 matching lines @@
                     kindsWithPendingClasses.contains(template));
 
       if (!pendingStuff) {
         // If there is pending stuff, or the compiler was cancelled, we
         // shouldn't reuse the compiler.
         cachedCompiler = compiler;
       }
     });
   }).then((_) => cachedCompiler);
 }