Expose Service Describer in Dart mojo package

mojo/dart/apptests/test_apps/echo_with_service_describer/lib/main.dart

+// 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 'dart:async';
+
+import 'package:mojo/application.dart';
+import 'package:mojo/bindings.dart';
+import 'package:mojo/core.dart';
+import 'package:_mojo_for_test_only/test/echo_service.mojom.dart';
+
+/// DummyEchoServiceImpl is unused in the corresponding test_app.
+class DummyEchoServiceImpl implements EchoService {
+  dynamic echoString(String value, [Function responseFactory]) => null;
+
+  dynamic delayedEchoString(String value, int millis,
+      [Function responseFactory]) => null;
+
+  void quit() {}
+}
+
+class EchoApplication extends Application {
+  EchoApplication.fromHandle(MojoHandle handle) : super.fromHandle(handle);
+
+  @override
+  void acceptConnection(String requestorUrl, String resolvedUrl,
+      ApplicationConnection connection) {
+    connection.provideService(EchoService.serviceName,
+      (endpoint) => new DummyEchoServiceImpl(),
+      description: EchoServiceStub.serviceDescription);
+  }
+}
+
+main(List args) {
+  MojoHandle appHandle = new MojoHandle(args[0]);
+  String url = args[1];
+  new EchoApplication.fromHandle(appHandle)
+    ..onError = ((_) {
+      MojoHandle.reportLeakedHandles();
+    });
+}