PreviewShell class

A project's catalog shell: the chrome every demo is rendered inside, and the one place its top-bar switches are declared.

// demo/shell.dart
Widget wrapInApp(Widget child) => PreviewShell(
  'app',
  builder: (context, axes) => MyApp(
    flavor: axes.picker('flavor', {
      'Dev': Flavor.dev,
      'Production': Flavor.prod,
    }, Flavor.dev),
    home: child,
  ),
);

A preview names it the way it always did, with @Preview(wrapper: wrapInApp). Nothing about the shell is discovered: it is an ordinary Widget Function(Widget), so Flutter's own previewer and the real app call it unchanged and PreviewAxes answers them with the defaults.

id is what the host files selections under, so leaving a shell and coming back finds the flavour you had chosen. Write it once and leave it alone: it is a name rather than a path, so moving or renaming the file does not lose what was set.