code: function startAppServer(fn) {
    if (!checkingAppServerPort) {
      server = fork(appServer.file, [], appServer.options);
      server.on('exit', code => {
        checkingAppServerPort = false;
        server.removeAllListeners();
        server = null;
        fn(Error('failed to start server'));
      });
  
      checkingAppServerPort = true;
      waitForPortOpen(appServer.port, fn);
    }
  }
docs: Start app server
  @param {Function} fn(err)