Sadly, there isn’t a backdrop widget in the official material package.
This is true, but there are official examples:
- https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/gallery/backdrop.dart
- https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/demo/material/backdrop_demo.dart
You can build and run the Flutter Gallery app and check the backdrop demo:
