expand_stylesheet_sources

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 580
        def expand_stylesheet_sources(sources)
          if sources.first == :all
            @@all_stylesheet_sources ||= Dir[File.join(STYLESHEETS_DIR, '*.css')].collect { |file| File.basename(file).gsub(/\.\w+$/, '') }.sort
          else
            sources.collect do |source|
              determine_source(source, @@stylesheet_expansions)
            end.flatten
          end
        end