…` SDK Version.

The main thread can easily look these up by ID, and in HTML5, sending the full `JobData` can cause errors.

`__activeThreads` and `__idleThreads` only need to be allocated for multi-threaded pools. Plus, there's no benefit to using a `List` here; we only add to and remove from the end.

And finally, checking `event.job == null` instead of `isOfType()` is faster and avoids an issue in HTML5. Sadly it is less safe, so we might need to revisit it eventually.

It looks like we'll want to take `BackgroundWorker` in a different direction, so for the moment it's safest not to change anything about it. That way, there's only one historical version to maintain backwards compatibility with.

As with `BackgroundWorker`, we're postponing major changes to give us more time to consider.

While I put a lot of effort into that guide, we're changing several things suddenly, and I don't have time to make sure it's up to date.

Restore old `Future` and `BackgroundWorker` behavior.