1. @override
void warn(PackageWarning kind, { String message, Locatable referredFrom })

Source 

@override
void warn(PackageWarning kind, {String message, Locatable referredFrom}) {
  if (enclosingCombo != null) {
    enclosingCombo.warn(kind, message: message, referredFrom: referredFrom);
  } else {
    super.warn(kind, message: message, referredFrom: referredFrom);
  }
}