code: function createRemoveNodeHandler(el, fn) {
    return function (mutations, observer) {
      mutations.forEach(function (mutation) {
        if (_includes(mutation.removedNodes, el)) {
          fn();
          observer.disconnect();
        }
      });
    };
  }
docs: Create a remove node handler that runs fn and removes the observer.
  @param {Element} el
  @param {Function} fn
  @returns {Function}