ES6 export keys need to be sorted

[Per the spec, modules have an [[Exports]] slot that is sorted:](https://www.ecma-international.org/ecma-262/6.0/#sec-module-namespace-exotic-objects)

> A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using Array.prototype.sort using SortCompare as comparefn.

[And enumerating over a module iterates over this list:](https://www.ecma-international.org/ecma-262/6.0/#sec-module-namespace-exotic-objects-enumerate)

> When the [[Enumerate]] internal method of a module namespace exotic object O is called the following steps are taken:
>
> Let exports be the value of O’s [[Exports]] internal slot.
> Return CreateListIterator(exports).