untrack method
Untracks element.
Implementation
void untrack(Element element) {
  var resizeObserver = _resizeObserver;
  if (resizeObserver != null) {
    _untrackResizeObserver(resizeObserver, element);
    return;
  }
  var trackElementValue = _trackElementValue;
  if (trackElementValue != null) {
    trackElementValue.untrack(element);
    return;
  }
  throw UnsupportedError("Can't track element resize");
}