test: test travis on earlier dart sdk (2.1 & 2.2) as well as stable and dev
test: test travis on earlier dart sdk (2.1 & 2.2) as well as stable and dev

tekartik/sembast.dart,alextekartik/sembast.dart,tekartik/sembast.dart,tekartik/sembast.dart,alextekartik/sembast.dart