Make JSXTransformer use async XHR

[zpao]

We were lazy before and just used sync XHR to make sure multiple scripts were still executed in the right order. However I'm not getting a warning in my console (Firefox Nightly) that says sync XHR is deprecated.

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help http://xhr.spec.whatwg.org/

Let's be preemptive and just build in the minimal work here to switch to async xhr and ensure we still process scripts in order.

[syranide]

Wouldn't you actively mess with expected behavior though? I.e, scripts might no longer have been loaded at onLoad?

[sophiebits]

With the JSX transformer, scripts are already not loaded at onload I think. We were using setTimeout 0 in a few places in our code at KA because of this (now integrated properly with our build process so it's not necessary).

[zpao]

Yea, it's already broken and there's nothing we could do about it really so we're no worse off.

[sophiebits]

Fixes by #1558.