Expose Service Describer in Dart mojo package

mojo/dart/apptests/test_apps/BUILD.gn

 # Copyright 2014 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/tools/bindings/mojom.gni")
 
 group("test_apps") {
   deps = [
     ":echo_service_interface",
     ":pingpong_service_interface",
     "//mojo/dart/apptests/test_apps/echo",
+    "//mojo/dart/apptests/test_apps/echo_with_service_describer",
     "//mojo/dart/apptests/test_apps/pingpong",
     "//mojo/dart/apptests/test_apps/pingpong_target",
   ]
 }
 
 mojom("echo_service_interface") {
   sources = [
     "interfaces/echo_service.mojom",
   ]
 }
 
 mojom("pingpong_service_interface") {
   sources = [
     "interfaces/pingpong_service.mojom",
   ]
 }