Key type for NativeAssocArray

Can we parametrize the key type of NativeAssocArray?

I'm trying to create an extern for a "Collection" interface:
```
extern interface CollectionInterface {.
function toArray():NativeArray;
}
```
NativeArray isn't super useful since I lose the value type.
NativeIndexedArray isn't right since the key type may be String.
I can't use NativeAssocArray right now since the key type may be Int.