default form helper

On line 1152 of form_helper.rb, there is this line:

        def default_form_builder
          builder = ActionView::Base.default_form_builder
          builder.respond_to?(:constantize) ? builder.constantize :
builder
        end

I tried searching for default_form_builder on ActiveView::Base but
couldnt find the definition. Can anyone guide to where it's defined,
specifically file and line number?