getSubValues method
Return sub values of key.
Implementation
List<V> getSubValues(K? key, {bool includePurgedEntries = false}) {
  var subValues = <V>[];
  if (key == null) return subValues;
  if (includePurgedEntries) {
    _getSubValuesImplIncludePurgedEntries(key, subValues);
  } else {
    _getSubValuesImpl(key, subValues);
  }
  return subValues;
}