Ensure that local exports have preference over export *

Whenever an `export *` statement is used, the variable `$__exportNames` is declared as the array of known export names. Before updating a binding for an `export *` item, we first check against this list and skip if already present.
Sorry I'm not following this. The comment claims this is generated by the code, but the code says something more like
```js
var $__exportNames = {'b': true};
```