Why ActionController::Base.class_eval for includes?

Just out of curiosity, why does action_controller.rb include classes
with class_eval instead of directly? I.e., why

ActionController::Base.class_eval do
  include ActionController::Flash
  include ActionController::Filters
  ...
end

instead of just

class ActionController::Base
  include ActionController::Flash
  include ActionController::Filters
  ...
end