There is no guide or docs showing how to implement a App Widget for a flutter app. It is definitely possible to implement a app widget with native code. Just create a flutter project and open the android part with android studio, just implement your home screen widget, it'll work like a charm.
I just wrote a simple app widget for a flutter app that takes you to https://flutter.io link when tapped. Here is the simple app created with flutter and a app widget added with native code. Install it and look for home_widget_example on Home Screen Widgets.
If you wanted to look into the source code, and build app from source. Here it is.
