Create an apptesting framework for dart.

services/dart/dart_apptests/BUILD.gn

 # 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("//mojo/public/dart/rules.gni")
 
-dart_packaged_application("pingpong") {
-  output_name = "dart_pingpong"
+dart_packaged_application("dart_apptests") {
   sources = [
     "main.dart",
+    "echo_apptests.dart",
+    "pingpong_apptests.dart",
   ]
   deps = [
+    "//mojo/dart/apptest",
+    "//services/dart/test:echo_service_interface",
     "//services/dart/test:pingpong_service_interface",
-    "//mojo/public/interfaces/application",
   ]
   strict = is_debug
 }