import 'package:http/http.dart' as http;
import 'dart:convert';
import 'dart:async';

Future changePassword(String newPassword) async {
  const String API_KEY = 'YOUR_API_KEY';
  final String changePasswordUrl =
      'https://www.googleapis.com/identitytoolkit/v3/relyingparty/setAccountInfo?key=$API_KEY';

      final String idToken = await user.getIdToken(); // where user is FirebaseUser user

    final Map payload = {
      'email': idToken,
      'password': newPassword,
      'returnSecureToken': true
    };

  await http.post(changePasswordUrl, 
    body: json.encode(payload), 
    headers: {'Content-Type': 'application/json'},  
  )
}