toJson method
Returns a JSON object representation of this instance.
Implementation
@override
 Map<String,dynamic> toJson() {
  Map<String,dynamic> data = super.toJson();
  data["uuid"] = uuid;
  data["holes"] = [];
  for (int i = 0, l = holes.length; i < l; i++) {
    final hole = holes[i];
    data["holes"].add(hole.toJson());
  }
  return data;
}