I had this lint error in several of my http json decode responses. The response was a simple list:
http response [{"total":2}]
Using List.from() was the solution. You can read about it here:
My final code:
final List<dynamic> result = jsonDecode(response.body) as List<dynamic>;
final List<Map<String, dynamic>> item =
List<Map<String, dynamic>>.from(result);
final int totalComments = int.parse(item[0]['total'].toString());