Re: [whatwg/streams] `.throw()`ing an async iterator (Issue #1284)

In my iteration utils I have an abort routine that `throw()`s first then `return()`s if it doesn't take. This way I can deliver the error if the iterator wants it, but still ensure it closes no matter what happens.

```js
function abort (iterator, error) {
  try {
    const result = iterator.throw(error)
    if (result.done) return
  } catch {
    return
  }
  iterator.return(error)
}
```

But this would change the semantics of iterator use right at the core of the language. That would be a big change.

I was surprised to see loops don't deliver the error, but then I realized just what you said. It may actually not close the iterator, so you really need the `return()` in there somewhere.

There's that additional complication that `throw()` could actually throw a completely new error. I just drop that one, which is kind of awkward.

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