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

services/dart/dart_apptests/pingpong_apptests.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.
 
 library pingpong_apptests;
 
 import 'dart:async';
 
 import 'package:apptest/apptest.dart';
-import 'package:dart/test/pingpong_service.mojom.dart';
+import 'package:mojom/dart/test/pingpong_service.mojom.dart';
 import 'package:mojo/public/dart/application.dart';
 import 'package:mojo/public/dart/bindings.dart';
 import 'package:mojo/public/dart/core.dart';
 
 class _TestingPingPongClient extends PingPongClient {
   final PingPongClientStub stub;
   Completer _completer;
 
   _TestingPingPongClient.unbound() : stub = new PingPongClientStub.unbound() {
     stub.impl = this;
@@ skipping 85 matching lines @@
       targetServiceProxy.ptr.ping(100);
       pongValue = await pingPongClient.waitForPong();
       expect(pongValue, equals(101));
 
       await pingPongClient.stub.close();
       await targetServiceProxy.close();
       await pingPongServiceProxy.close();
     });
   });
 }