emit generators when targeting es5

[marzelin]

TypeScript Version: 2.2.1

When I use generators in my code while targeting es5, the following error appears:
'Generators are only available when targeting ECMAScript 2015 or higher.'
but tslib already provides a generator helper (it's used when converting async functions).
It would be nice if typescript was able to emit bare generators as well.

[mhegazy]

This should be coming in TS 2.3 under ---downlevelIteration flag. You should be able to try it out in typescript@next today.

[mhegazy]

Duplicate of #1564. Fixed by #12346