lib/jsdom/living/helpers/shadow-dom.js
function assignSlotableForTree(root) {
  for (const slot of domSymbolTree.treeIterator(root)) {
    if (isSlot(slot)) {
      assignSlotable(slot);
    }
  }
}
function assignSlotableForTree(root) {
  for (const slot of domSymbolTree.treeIterator(root)) {
    if (isSlot(slot)) {
      assignSlotable(slot);
    }
  }
}
https://dom.spec.whatwg.org/#assign-slotables-for-a-tree