void addKey(String keySystem, Uint8List key, [Uint8List initData, String sessionId])

Source 

void addKey(String keySystem, Uint8List key, [Uint8List initData, String sessionId]) {
  if (initData != null) {
    _blink.BlinkHTMLMediaElement.instance.webkitAddKey_Callback_4_(this, keySystem, key, initData, sessionId);
    return;
  }
  _blink.BlinkHTMLMediaElement.instance.webkitAddKey_Callback_2_(this, keySystem, key);
  return;
}