def descendants(include_singletons = false)
      return _inherited_by.map { |c| [c, c.descendants] }.flatten.uniq if BBLib.in_opal?
      ObjectSpace.each_object(Class).select do |c|
        (include_singletons || !c.singleton_class?) && c < self
      end
    end
def descendants(include_singletons = false)
      return _inherited_by.map { |c| [c, c.descendants] }.flatten.uniq if BBLib.in_opal?
      ObjectSpace.each_object(Class).select do |c|
        (include_singletons || !c.singleton_class?) && c < self
      end
    end
Return all classes that inherit from this class