Fix iterator closing around for-await-of and async-from-sync

[andreasrosdal]

Two halves of the same story: who closes the sync iterator when a for await step rejects. Today the loop closes it (it should not) and the async-from-sync wrapper does not (it should). Each commit alone moves a test in the wrong direction, so they are sent together.

1. for-await-of must not close on an abrupt next()

ForIn/OfBodyEvaluation calls next(), awaits the result, and on an abrupt completion sets iteratorRecord.[[Done]] and returns it without closing the iterator — an iterator that failed to produce a result is not closed.

The for-await-of bytecode keeps the iterator live in the slot the loop's catch offset unwinds to, so a next() that throws, or an awaited result that rejects, runs the trailing OP_iterator_close and calls return():

let returnCalled = 0;
const it = { [Symbol.asyncIterator]: () => ({
  next() { return Promise.reject(new Error('boom')); },
  return() { returnCalled++; return Promise.resolve({done: true}); },
})};
try { for await (const v of it) {} } catch (e) {}
returnCalled   // 1; V8: 0

Two opcodes bracket the next()+await sequence. OP_for_await_of_dup moves the iterator out of the slot the handler reads and parks it above the catch offset, so an abrupt result unwinds to an undefined iterator and the close is skipped; OP_for_await_of_restore puts it back once a result has been obtained. They are appended at the end of the opcode list so no opcode already embedded in a precompiled bytecode blob is renumbered.

2. The async-from-sync wrapper must close on a rejected value

AsyncFromSyncIteratorContinuation takes a closeOnRejection flag — true for %AsyncFromSyncIteratorPrototype%.next and .throw, false for .return. When it is set and the sync result is not done, the wrapped value promise gets an onRejected that performs IteratorClose(syncIteratorRecord, ThrowCompletion(error)) before the rejection reaches the consumer:

13. Else,
  a. Let closeIterator be a new Abstract Closure ... performs
     IteratorClose(syncIteratorRecord, ThrowCompletion(error)).
  b. Let onRejected be CreateBuiltinFunction(closeIterator, 1, "", « »).
14. Perform PerformPromiseThen(valueWrapper, onFulfilled, onRejected,
    promiseCapability).

js_async_from_sync_iterator_do() always passes JS_UNDEFINED as onRejected, so a sync iterator that yields a rejected promise is never closed. The same applies when PromiseResolve on the value itself throws.

Testing

suite	before	after
built-ins/AsyncFromSyncIteratorPrototype	6/38	0/38
language/statements/for-await-of	0/1234	0/1234
language/statements/for-of	0/751	0/751
language/expressions/async-generator	0/623	0/623

The six AsyncFromSyncIteratorPrototype entries are dropped from test262_errors.txt.

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