Flutter - Remove escape sequence in dart

json.decode can decode single strings too, so you should be able to just call it twice. The first time it'll return you a string (where the escape characters have been decoded) and the second time it'll decode that string into the map:

import 'dart:convert';

void main() {
  var a = r'''"{\"Response\" : {\"Responsecode\" : \"0\" , \"Response\" : \"Success\"}}"''';
  var b = json.decode(json.decode(a));
  print(b['Response']['Responsecode']); // 0
  print(b['Response']['Response']); // Success
}