contains method

@override bool contains(String? varName)

Check if this struct has the key in its own _fields or its prototypes' _fields

Implementation

@override
bool contains(String? varName) {
  if (varName == null) {
    return false;
  }

  if (_fields.containsKey(varName)) {
    return true;
  } else if (prototype != null && prototype!.contains(varName)) {
    return true;
  } else {
    return false;
  }
}