如何在flutter应用中启用firebase中的持久化存储？

apply plugin: 'com.google.gms.google-services'

import 'package:firebase_core/firebase_core.dart';

class FirebaseService {
  static Future<void> initialize() async {
    await Firebase.initializeApp();
  }
}

import 'package:flutter/material.dart';
import 'package:your_app/firebase_service.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await FirebaseService.initialize();
  runApp(MyApp());
}

import 'package:cloud_firestore/cloud_firestore.dart';

final FirebaseFirestore firestore = FirebaseFirestore.instance;

void addData() {
  firestore.collection('users').add({
    'name': 'John',
    'age': 25,
  });
}

void getData() {
  firestore.collection('users').get().then((QuerySnapshot snapshot) {
    snapshot.docs.forEach((DocumentSnapshot document) {
      print(document.data());
    });
  });
}