with_output_file (ActiveSupport::Testing::Benchmarker)

    # File 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