_addToggleButton(div, hidden = false) {
    const toggler = document.createElement("div");
    toggler.className = "treeItemToggler";
    if (hidden) {
      toggler.classList.add("treeItemsHidden");
    }
    toggler.onclick = evt => {
      evt.stopPropagation();
      toggler.classList.toggle("treeItemsHidden");

      if (evt.shiftKey) {
        const shouldShowAll = !toggler.classList.contains("treeItemsHidden");
        this._toggleTreeItem(div, shouldShowAll);
      }
    };
    div.prepend(toggler);
  }
Prepend a button before a tree item which allows the user to collapse or
expand all tree items at that level; see `_toggleTreeItem`.
@param {HTMLDivElement} div
@param {boolean|object} [hidden]
@protected