Down-level destructuring in `for..in` statement

[rbuckton]

We report an error for the following code, :

for (var {toString} in { a: 1 }) { // error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
   console.log(toString);
}

We also emit the following incorrect javascript:

for (var toString = (void 0).toString in { a: 1 }) {
    console.log(toString);
}

We should instead emit:

for (var _a in { a: 1 }) {
    var toString = _a.toString;
    console.log(toString);
}

While it is unlikely this will be very useful, this is another of the ES6 compatibility tests on the kangax compatibility table that we could consider addressing as it seems a small enough change.