structInit creates a "non-exist" but typeable constructor

```haxe
class Main{
static function main() {
$type(Foo.new);
}
}

@:structInit
class Foo {
@:optional var foo:Int;
}
```

```
src/Main.hx:3: characters 9-16 : Foo does not have a constructor
src/Main.hx:3: characters 9-16 : Warning : (foo : Null) -> Foo
```