Member alias using type arguments fails at runtime

[chochos]

class C<A,B>(a,b) {
  shared A a;
  shared B b;
  alias E => A->B;
  shared void test() {
    Object e = key->item;
    if (is E e) {
      print(e)
    }
  }
}

Invoking test() fails because the alias E has a reference to type arguments of C but the js for the alias only refers to $$targs$$.