Let async generators directly await an incoming promise

[Simo Costa]

Currently the following illustrative syntax is forbidden:
```js
obj = {
    async*[Symbol.asyncIterator]() {
        const res = await yield;
        // do stuff with res
     }
}
```
Used in this way (simple example):
```js
const ait = [Symbol.asyncIterator]();

ait.next();
ait.next(p); // p is a Promise
```

We cannot directly write `await yield`. Is there any particular reason?
We have to write:
```js
obj = {
    async*[Symbol.asyncIterator]() {
        const p = yield;
        const res = await p;
        // do stuff with res
     }
}
```




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