Async arrow function being `undefined`

[Kanguk Lee]

Hello,

`x` is `undefined` after executing `let x = async x => { for await ( { x } of [ 0 ] ) ; } ; x ( ) ;` in WebKit JSC,
but I think that it is expected to be `async x => ....`:

---
$ jsc
>>> let x = async x => { for await ( { x } of [ 0 ] ) ; } ; x ( ) ;
[object Promise]
>>> x
undefined
>>>
---

while other engines behave like:

---
# V8
V8 version 10.9.0
d8> let x = async x => { for await ( { x } of [ 0 ] ) ; } ; x ( ) ;
[object Promise]
d8> x
async x => { for await ( { x } of [ 0 ] ) ; }
d8>
---


WebKit version: 615.1.10

[Yusuke Suzuki]

It is JSC shell's behavior that `let` is scoped to this line.
So, it is not defined as a standard.