code: function portChecker( port, callback ) {
    return detect( port || DEFAULT_PORT ).then(( port_ ) => {
      if ( port_ !== port ) {
        print();
        print( chalk.yellow( `Something is already running on port ${port}.` ));
        print();
      } else {
        callback();
      }
    }).catch(() => {
      process.exit( 1 );
    });
  }
docs: 检测端口是否被占用