Unification change

Strangely, while tackling #4001 , it seems that the following is invalid:

``` haxe
class Main
{
static function main()
{
var x:cs.NativeArray<{}> = new cs.NativeArray(1);
}
}
```

This fails now with:

```
Main.hx:5: characters 2-60 : cs.NativeArray should be cs.NativeArray<{ }>
Main.hx:5: characters 2-60 : Type parameters are invariant
Main.hx:5: characters 2-60 : Dynamic should be { }
```

Not that I'm advocating for `` to be how it was. But this was a recent change, wasn't it? Is this intentional?