add method
Adds a validator to the appropriate position in the chain. Pre-coercion validators go before coercion, post-coercion validators go after.
Implementation
void add(IValidator v) {
  if (_hasCoercion) {
    _postValidators = _postValidators == null ? v : (_postValidators! & v);
  } else {
    _preValidators = _preValidators == null ? v : (_preValidators! & v);
  }
}