hashCode property
Deep hashCode.
A BuiltSet is only equal to another BuiltSet with equal elements in
any order. Then, the hashCode is guaranteed to be the same.
Implementation
@override
int get hashCode {
  _hashCode ??= hashObjects(
      _set.map((e) => e.hashCode).toList(growable: false)..sort());
  return _hashCode!;
}