Return value of waitUntil()?

[inexorabletash]

Options include:

1. a Promise dependent on the promise the transaction is waiting on
2. just whatever is passed in
3. undefined

1 is redundant with .complete
2 is not terribly helpful
3 is what ExtendableEvent is defined as

So 3 probably makes the most sense and leaves room to change it in the future.

[inexorabletash]

In email to public-webapps, @domenic suggests returning tx.complete as a convenience.

[inexorabletash]

As hinted at in #9 it may be convenient to make it a pass-through so you can write e.g.

  const url = await tx.objectStore('urls').get(key);
  const request = await tx.waitUntil(fetch(url));
  const text = await tx.waitUntil(request.text());
  tx.objectStore('bodies').put(text, key);

.... which looks better than the "IIAFE" alternative:

tx.waitUntil((async () => {
  const url = await tx.objectStore('urls').get(key);
  const request = await fetch(url);
  const text = await request.text();
  await tx.objectStore('bodies').put(text, key);
})());