subscribe 方法

实现

Future<PushSubscription> subscribe([Map? options]) {
  var options_dict = null;
  if (options != null) {
    options_dict = convertDartToNative_Dictionary(options);
  }
  return promiseToFuture<PushSubscription>(
      JS("creates:PushSubscription;", "#.subscribe(#)", this, options_dict));
}