Dart: add value to Map conditionally by using spread

gistfile1.txt

void main() {
  bool test = false;
  Map<String, String> myMapA = {
    "a": "1",
    "b": "2",
   ... test == true ? {"c":"3"} : {}
  };
    Map<String, String> myMapB = {
    "a": "1",
    "b": "2",
   ... test == false ? {"c":"3"} : {}
  };

  print(myMapA);
  print(myMapB);
}