`@:multiType`-inception instantiates non-existent constructors

I have tried implementing a custom map abstract via `@:multiType` and have encountered a weird issue:

```haxe
class Test {
static function main() {
var x = new ObservableMap(['1' => 2]);
}
}

@:multiType(@:followWithAbstracts K)
abstract ObservableMap(ObservableMapImpl) {
public function new(?init:haxe.Constraints.IMap);

@:to static function toIntMap(dict:ObservableMapImpl, ?init:haxe.Constraints.IMap):ObservableMapImpl
return null;
}

class ObservableMapImpl {

}
```

The [generated js code](https://try.haxe.org/#2FE6402e) will contain `new haxe_ds_Map().set("1",2);` which depending on `js_es` will crash in one way or another - and other targets produce corresponding errors too.

Side note: I also tried `Map` for the initial values, with the same result. My guess is that there's so many levels of type parameters, that the compiler somehow doesn't realize it must select an implementation.