code: static void addDetachObserver(HTMLElement element, ObserverCallback callback) {
          if (!ready) {
              startObserving();
          }
          detachObservers.add(createObserver(element, callback, DETACH_UID_KEY));
      }
docs: Check if the observer is already started, if not it will start it, then register and callback for when the
  element is removed from the dom.