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

`for..of` doesn't call `throw()` either.

```javascript
let it = {
    [Symbol.iterator]: () => it,
    next() {
        return { done: false, value: "a" };
    },
    throw(e) {
        console.log("it.throw() called");
        return { done: true };
    },
    return() {
        console.log("it.return() called");
        return { done: true };
    }
};
for (const elem of it) {
    throw new Error("boom!");
}
```
The above snippet logs:
```
it.return() called
Uncaught Error: boom!
```
So even if we added such a hook, it wouldn't do anything in your example. AFAIK the only built-in construct that interacts with `throw()` is `yield*`.

I don't see how we could ever make `throw()` work. With generators, `throw()` can still cause the generator to *resume normally*, as in the following example:
```javascript
function* gen() {
  while (true) {
    try {
      yield;
    } catch {
      continue;
    }
  }
}
```
But with `for..of`, throwing inside the loop body *must* propagate upwards. We cannot "resume iteration" afterwards. So we only have `return()`.

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