Dart: Reorganize files

mojo/dart/apptests/dart_apptests/lib/src/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:mojo_apptest/apptest.dart';
 import 'package:mojo/application.dart';
@@ skipping 99 matching lines @@
       targetServiceProxy.ptr.ping(100);
       pongValue = await pingPongClient.waitForPong();
       expect(pongValue, equals(101));
 
       await pingPongClient.stub.close();
       await targetServiceProxy.close();
       await pingPongServiceProxy.close();
     });
   });
 }