expand_stylesheet_sources

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 534
        def expand_stylesheet_sources(sources)
          if sources.first == :all
            @@all_stylesheet_sources ||= Dir[File.join(STYLESHEETS_DIR, '*.css')].collect { |file| File.basename(file).split(".", 1).first }.sort
          else
            sources
          end
        end