Property enumeration order of String exotic objects is wrong

Consider:

```js
let obj = Object('asdf');
obj.a = 1;
console.log(Reflect.ownKeys(obj).map(x => x.toString()).join(','));
```

Per the overload for [`[[OwnPropertyKeys]]` for String exotic objects](https://tc39.github.io/ecma262/#sec-string-exotic-objects-ownpropertykeys), and in accord with every other browser, this should print

```
0,1,2,3,length,a
```

But in fact it prints

```
0,1,2,3,a,length
```

See also [this open spec PR](https://github.com/tc39/ecma262/pull/1242) about very large integer indices and [this related ChakraCore issue](https://github.com/Microsoft/ChakraCore/issues/3396) about enumeration order for classes.