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我目前正在尝试使用 Flutter 制作一些移动代码。我正在尝试使用 gcloud 库 dart 将数据发布/订阅到 GCP Cloud Pub/Sub。这是 main.dart 的代码：

import 'dart:io';
import 'package:googleapis_auth/auth_io.dart' as auth;
import 'package:http/http.dart' as http;
import 'package:gcloud/db.dart';
import 'package:gcloud/storage.dart';


import 'package:gcloud/pubsub.dart';
import 'package:gcloud/service_scope.dart' as ss;
import 'package:gcloud/src/datastore_impl.dart' as datastore_impl;

Future<int> main() async {

  // Read the service account credentials from the file.
var jsonCredentials = new File('path/file.json').readAsStringSync();
var credentials = new auth.ServiceAccountCredentials.fromJson(jsonCredentials);

// Get an HTTP authenticated client using the service account credentials.
var scopes = []    
    ..addAll(PubSub.SCOPES);
var client = await auth.clientViaServiceAccount(credentials, …