`async` should be usable as an object key

[cdepillabout]

There was a bug reported when trying to use language-javascript-0.7.1.0 in PureScript: purescript/purescript#4199 (comment)

It appears that async is completely treated as a keyword, and not allowed as an object key. So something like myObj.async doesn't parse. But apparently this is allowed in JS.

This was probably introduced in #113.

(I'm not super familiar with language-javascript, or even JS, but maybe @MonoidMusician, @garyb, or @JordanMartinez could follow-up on this if @erikd needs more information or I've reported this incorrectly.)

[rhendric]

I think myObj.async expressions parse correctly, as do { async: 0 } expressions and similar. I believe the issue that PureScript encountered was actually with a function (async) { ... } expression; I tested and encountered problems with that and with similar constructions using arrow syntax.