es6.blockScopingTDZ creates invalid JS for for...in loop

[loganfsmyth]

let data = {};
let key;
for (key in data){
}

becomes

var _temporalAssertDefined = function (val, name, undef) { if (val === undef) { throw new ReferenceError(name + " is not defined - temporal dead zone"); } return true; };

var _temporalUndefined = {};

var data = _temporalUndefined;
var key = _temporalUndefined;
data = {};
key = undefined;
for (_temporalAssertDefined(key, "key", _temporalUndefined) && key in _temporalAssertDefined(data, "data", _temporalUndefined) && data) {}

Seems to be because the let key; is outside the for loop and used as a key.

[sebmck]

Fixed in the experimental branch.