getSync<T extends Object>  method 
Retrieves a synchronous dependency.
Implementation
Option<Sync<T>> getSync<T extends Object>({
  Entity groupEntity = const DefaultEntity(),
  bool traverse = true,
}) {
  assert(T != Object, 'T must be specified and cannot be Object.');
  UNSAFE:
  return get<T>(groupEntity: groupEntity, traverse: traverse).map(
    (e) => e.isSync()
        ? e.sync().unwrap()
        : Sync.err(Err('Called getSync() for an async dependency.')),
  );
}