compute_source_path

# File actionpack/lib/action_view/asset_paths.rb, line 34
    def compute_source_path(source, dir, ext)
      source = rewrite_extension(source, dir, ext) if ext

      sources = []
      sources << config.assets_dir
      sources << dir unless source[0] == //
      sources << source

      File.join(sources)
    end