* PopupMenuButton: create IconButton if child is Icon
Otherwise the resulting button has an abnormally small and rectangular
area. With multiple PopupMenuButton(child: Icon) they get squished
together in the AppBar.
* Add separate icon argument to PopupMenuButton
* Fix style issues and tweak dartdocs
* Add tests for icon argument to PopupMenuButton
* Group icon tests and fix broken test, analyzer warnings
* Test that the correct custom icon is present
* Apply De Morgan's to work around dart analyzer bug
see: https://github.com/dart-lang/sdk/issues/30288