Add withAppEngineService() to package:appengine

This will allow users to run background tasks which can use API services.

An example usage could look like:

  import 'dart:async';
  import 'dart:io';
  import 'package:appengine/appengine.dart';

  Future backgroundWork() {
    return dbService.query(Person).run().toList().then((List<Person> persons) {
      // Do something with `persons`.
    });
  }

  void mainHandler(HttpRequest request) {
    request.response..write('hello world')..close();
  }

  main() {
    withAppEngineServices(() {
      return Future.wait([
          runAppEngine(mainHandler),
          backgroundWork(),
      ]);
    });
  }

R=sgjesse@google.com, wibling@google.com

Review URL: https://codereview.chromium.org//775043002