elemental2.dom.Window is missing document variable

I'm trying to access the document from a elemental2.dom.HTMLIFrameElement but noticed that elemental2.dom.Window (via HTMLIFrameElement.contentWindow) has no document variable.
There is no chance to access an iframe document via elemental2.

This should be possible. Please see more details in the example at https://developer.mozilla.org/en-US/docs/Web/API/HTMLIFrameElement/contentWindow

I need to solve that via native gwt code:

```
public final native elemental2.dom.Document document(elemental2.dom.Window contentWindow) /*-{
return contentWindow.document;
}-*/;
```