Doesn't look like it is actually needed anywhere?

notes irrelevant to the proposal

Stubbed out src/harness/collectionsImpl.ts::closeIterator (which doesn't fail a single test lol) because whenever I tried to do something like

export function closeIterator<T>(iterator: Iterator<T> | Generator<T>) {
    // Iterators don't have to have a return method
    // eslint-disable-next-line no-in-operator
    if ("return" in iterator && typeof iterator.return === "function") iterator.return.call(iterator);
}

I would get test timeouts (flaky?)

Actually not sure how this is meant to be written according to the enforced styles, since truthiness doesn't narrow it, and typeof is not allowed on the | undefined field.
Object.prototype.hasOwnProperty definitely isn't appropriate here.