Dart: Put generated .mojom.dart files under mojoms pacakge.

services/dart/dart_apptests/main.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:apptest/apptest.dart';
-import 'package:dart/test/echo_service.mojom.dart';
+import 'package:mojom/dart/test/echo_service.mojom.dart';
 
 import 'echo_apptests.dart' as echo;
 import 'pingpong_apptests.dart' as pingpong;
 import 'io_apptests.dart' as io;
 
 main(List args) {
   runAppTests(args[0], [echo.echoApptests,
                         io.ioApptests,
                         pingpong.pingpongApptests]);
 }