static object builder

main.dart

void main() async {
  final cache = await Cache.createCache();
}

class Cache {
  static Db myDb;

  static createCache() async {
    if (myDb == null) {
      myDb = await Future.delayed(Duration(seconds: 1));;
    }
    
    return Cache._internal();
  }
  
  Cache._internal() {
    // this is a private constructor, so outside files can't use it
    
    // use myDb in here
  }
}

class Db {}