Fix issue with generating async functions inside loops.

[cristianoc]

Fixes #6444

The example comes from a code transformation that deals with closures inside loops. Specifically, since var is function scope, and immediately-executed function is generated to capture the variable.
The generated code was missing an async in the returned function, but also an await on the outside.

Note this entire transformation becomes unnecessary after #6102

[cristianoc]

@zth @cknitt can you think of other tests to try? Asking as this becomes the only test that triggers this issue, and 2 changes were required in the generated code, so making sure nothing is missing.

[zth]

Nothing I can think of OTOH. Landing let support is going to be great, so we can get rid of these things.