XS does not apply runtime strict mode restrictions to async functions

[jugglinmike]

For async functions defined in strict mode, XS does not enforce the relevant runtime restrictions:

$ ~/.jsvu/xs -e '"use strict"; (async function () { notStrict = true })().then(() => print("okay"));'
okay
$ ~/.jsvu/xs -e '"use strict"; (async () => { notStrict = true })().then(() => print("okay"));'
okay
$ ~/.jsvu/xs -e '"use strict"; (async () => notStrict = true)().then(() => print("okay"));'
okay
$ ~/.jsvu/xs -e '"use strict"; (async function () { "".notStrict = true })().then(() => print("okay"));'
okay
$ ~/.jsvu/xs -e '"use strict"; (async () => { "".notStrict = true })().then(() => print("okay"));'
okay
$ ~/.jsvu/xs -e '"use strict"; (async () => "".notStrict = true)().then(() => print("okay"));'
okay

This is also the case when the function itself enables strict mode:

$ ~/.jsvu/xs -e '(async function () { "use strict"; notStrict = true })().then(() => print("okay"));'
okay

...but XS does correctly apply syntactic restrictions:

$ ~/.jsvu/xs -e '"use strict"; (async () => static)().then(() => print("okay"));'
SyntaxError: missing expression

XS correctly enforces strict mode in synchronous functions:

$ ~/.jsvu/xs -e '"use strict"; (() => notStrict = true)();'
ReferenceError: ?: set notStrict: undefined variable
$ ~/.jsvu/xs -e '"use strict"; (() => { notStrict = true })();'
ReferenceError: ?: set notStrict: undefined variable
$ ~/.jsvu/xs -e '"use strict"; (function () { notStrict = true })();'
ReferenceError: ?: set notStrict: undefined variable

Interestingly, strict mode is also correctly applied for synchronous functions defined within the erroneously-non-strict async functions:

$ ~/.jsvu/xs -e '"use strict"; (async () => { (() => notStrict = true)(); })().then(() => print("okay"), (err) => print(err));' 
ReferenceError: ?: set notStrict: undefined variable

This behavior was observed using the xst binary as provided by jsvu.

$ ~/.jsvu/xs -v
XS 10.0.0
$ uname -a
Linux bruce 4.15.0-88-generic #88-Ubuntu SMP Tue Feb 11 20:11:34 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux