Emit destructuring patterns in 'for...in' statements for ES3/5

[JsonFreeman]

The following is allowed by the ES6 grammar:

for (var [a, b] in []) { }

This results in a type check error, because it doesn't make sense to destructure a string key. But it should still emit correctly downlevel. Right now we emit:

for (var _a = void 0, a = _a[0], b = _a[1] in []) { }

which is incorrect.

[mhegazy]

so what is the proposed emit?

[JsonFreeman]

Similar to 'for...of', but 'for...in'-ified:

for (var _a in []) {
    var a = _a[0], b = _a[1];
}

[RyanCavanaugh]

Doesn't seem like anyone has noticed this