TextTrack addTextTrack(String kind, [ String label, String language ])

Source 

TextTrack addTextTrack(String kind, [String label, String language]) {
  if (language != null) {
    return _blink.BlinkHTMLMediaElement.instance.addTextTrack_Callback_3_(this, kind, label, language);
  }
  if (label != null) {
    return _blink.BlinkHTMLMediaElement.instance.addTextTrack_Callback_2_(this, kind, label);
  }
  return _blink.BlinkHTMLMediaElement.instance.addTextTrack_Callback_1_(this, kind);
}