Expose Service Describer in Dart mojo package

mojo/dart/apptests/test_apps/echo_with_service_describer/BUILD.gn

-# Copyright 2015 The Chromium Authors. All rights reserved.
+# Copyright 2016 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("device_info") {
+dart_pkg("echo_with_service_describer") {
   apps = [ [
-        "dart_device_info",
+        "dart_echo_with_service_describer",
         "lib/main.dart",
       ] ]
   sources = [
     "pubspec.yaml",
   ]
   deps = [
     "//mojo/dart/packages",
     "//mojo/public/dart",
   ]
 }