This is an alternative to #2818. See that PR for more context - this implements option 3 of the options listed there. Fixes #2813.

The main effect of this PR would be that if you had

let done = false;
let inner = {
  [Symbol.asyncIterator]: () => ({
    next() {
      if (done) {
        return Promise.resolve({ done: true });
      }
      done = true;
      return Promise.resolve({ done: false, value: Promise.resolve(0) });
    },
  }),
};

async function* outer() {
  yield* inner;
}

(async () => {
  for await (let x of outer()) {
    console.log(x);
  }
})().catch(e => {
  console.log('threw', e);
});

then you would see printed Promise.resolve(0) instead of, as currently, 0. In other words, it would behave as if you'd written for await (let x of inner) instead of of outer().

The other effect is a reduction in the number of promise ticks.

No difference (except in timing) would be observable without manually implementing Symbol.asyncIterator because async generators (and the async-from-sync wrapper, see step 6) already await values before yielding or returning them.