When return() is called on a generator that is in the process of yield* delegation, it should not only call return() on the delegated iterable, but should also exit the current generator. The __asyncDelegator processes each call in two steps: first it indicates to the outer generator that it should await the result of a call to next, throw, or return, then it returns the result of that await operation in a tick/tock fashion.

When the delegator "ticks", it returns an IteratorResult that looks like: { value: ["await", result], done: false }. The outer async generator will handle the await and call back into the delegator ("tock"). When the delegator "tocks", it processes the iterator result and either exits with a done: true or yields with a { value: ["yield", value], done: false }.

The problem lies in the fact that when the delegator "ticks" as part of a call to return(), it returns done: false. This informs the underlying generator runtime that the delegator did not actually close, so it continues processing. This change now unconditionally causes the delegator to return by always returning done: true when calling return() on an async delegator.

This change also awaits the result of an async delegator (the x in x = yield* y) to more closely align with the async iteration proposal.

Fixes #15471