with_output_file

# File activesupport/lib/active_support/testing/performance.rb, line 148
          def with_output_file
            fname = output_filename

            if new = !File.exist?(fname)
              FileUtils.mkdir_p(File.dirname(fname))
            end

            File.open(fname, 'ab') do |file|
              file.puts(HEADER) if new
              yield file
            end
          end