another scoping issue with conditions

[gavinking]

The fix to #430 was incomplete. Consider:

void z(Void o) {
    if (is Object o) {
        class Inner() {
            Object x = o;
        }
    }
    assert (is Object o);
    class Inner() {
        Object x = o;
    }
}

This well-typed code results in two errors:

specified expression must be assignable to declared type of x: Void is not assignable to Object