I believe this should return a `Future<HtmlWindow>`, where `HtmlWindow` is the class defined right now in `app.dart` (and returned as the `AppWindow.contentWindow`).

I think we should move `HtmlWindow` to either its own library or to `common.dart` so we can share it among the APIs.  I also will use it from `webview` once I add that wrapper, and there might be other API that needs to expose a similar type?