False positive unused_element in Flutter

[davidmartos96]

Describe the issue
When using latest Flutter 3.0.5, the linter warns about the key property as not being used on private widgets.

To Reproduce
With Flutter 3.0.5 run flutter create demo
Edit main.dart as in

Expected behavior
I would expect key to be blacklisted from the lint unused_element for both private and public widgets. That is, no warnings expected from this example.
It is never known beforehand when you would need to include a key as a consumer of a widget. That could include private widgets as well.

[davidmartos96]

Duplicate #49025.