Fix Array.fromAsync iterator close semantics

[littledivy]

Array.fromAsync closed its async iterator in a finally block, which calls return() even when the iterator completed normally, and never awaits the result. The spec only closes the iterator on an abrupt completion (IfAbruptCloseAsyncIterator, from the element await or the mapper), leaves an exhausted iterator alone, and awaits the return() result before the returned promise settles.

const iter = {
  [Symbol.asyncIterator]() {
    let i = 0;
    return {
      async next() { return { value: i, done: i++ > 0 }; },
      async return() { console.log("return()"); return { done: true }; },
    };
  },
};
await Array.fromAsync(iter);   // before: logs "return()"   after: no close

case	before	after
iterator exhausted (done: true)	return() called	not called
mapper throws	return() fire-and-forget	awaited before rejection
return() rejects after mapper throws	unhandled rejection	original error kept

Disclaimer: This is an AI-assisted patch from the v8x project.

[bnoordhuis]

Can you upstream the test to test262? Then we can be sure all engines implement identical behavior.

[littledivy]

Can you upstream the test to test262? Then we can be sure all engines implement identical behavior.

Opened PR tc39/test262#5095 and V8 CL https://chromium-review.googlesource.com/c/v8/v8/+/8140683