The promise returned by `@@dispose` should not be awaited

[nicolo-ribaudo]

When the language uses synchronous iterators as fallbacks for asynchronous iterators (using https://tc39.es/ecma262/#sec-async-from-sync-iterator-objects), it doesn't await the promises returned by next():

let done = false;

const it = {
  [Symbol.iterator]() {
    return {
      next() {
        let p = Promise.resolve({ value: 1, done });
        p.value = 2;
        p.done = done;
        done = true;
        return p;
      }
    }
  }
}

for await (const v of it) {
  console.log(v); // logs 2; if you replace @@iterator with @@asyncIterator it logs 1.
}

As such, if when using await using there is no @@asyncDisposable and thus we use the @@disposable fallback, if it returns a promise it should not be awaited.

The proposal spec doesn't distinguish between the method store in @@asyncDisposable and the one stored in @@disposable (steps 1.a and 1.b.i of https://tc39.es/proposal-async-explicit-resource-management/#sec-getdisposemethod).

I found this inconsistency while implementing this proposal in Babel: babel/babel#15633.

[nicolo-ribaudo]

The solution is probably to wrap the @@dispose method in another function to make sure that its return value is ignored.