featureIn function
The feature with this id, in this spec. Throws when nothing has it — an id that names nothing is a typo, not a null.
Takes the spec rather than reading a global, because the gallery has none: the app binds this to its own spec and exports the one-argument form its call sites already use.
Implementation
SettingFeature featureIn(List<SettingGroup> spec, String id) =>
spec.expand((g) => g.features).firstWhere((f) => f.id == id);