Expose Service Describer in Dart mojo package

mojo/dart/apptests/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_pkg("dart_apptests") {
   apps = [ [
         "dart_apptests",
         "lib/main.dart",
       ] ]
   sources = [
     "lib/src/connect_to_loader_apptests.dart",
     "lib/src/echo_apptests.dart",
     "lib/src/file_apptests.dart",
     "lib/src/io_http_apptests.dart",
     "lib/src/io_internet_address_apptests.dart",
     "lib/src/pingpong_apptests.dart",
+    "lib/src/service_describer_apptests.dart",
     "lib/src/uri_apptests.dart",
     "lib/src/versioning_apptests.dart",
     "pubspec.yaml",
   ]
   deps = [
+    "//mojo/dart/apptests/test_apps",
     "//mojo/dart/packages",
     "//mojo/public/dart",
-    "//mojo/dart/apptests/test_apps",
   ]
 }