问Flutter什么是正确的返回方法类型？

EN

List customerItems = await _helper.get('/Customers/v1/GetItems/$cid/$pfType/$targetId');

var groupByCustomerItems = groupBy(customerItems, (obj) => obj['itemtype']).map((key, value) => MapEntry(
    key,
    value.map((item) {
      return new CustomerItem.fromJson(item);
    }).toList()));

{Device: [Instance of 'CustomerItem', Instance of 'CustomerItem'], Dial-Up: [Instance of 
'CustomerItem', Instance of 'CustomerItem'], Domain: [Instance of 'CustomerItem'], EMAIL:             
[Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', 
Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', Instance 
of 'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 
'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 
'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem'], HSI: [Instance of 
'CustomerItem'], vo.phone: [Instance of 'CustomerItem', Instance of 'CustomerItem', Instance 
of 'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 
'CustomerItem', Instance of 'CustomerItem', Instance of 'CustomerItem', Instance of 
'CustomerItem']}

Future<List<CustomerItem>>