code: function writeCriticalFile(filePath, css) {
    _fs2.default.writeFile(filePath, css, { flag: append ? 'a' : 'w' }, function (err) {
      append = true;
      if (err) {
        console.error(err);
        process.exit(1);
      }
    });
  }
docs: Write a file containing critical CSS.
  
  @param {string} filePath Path to write file to.
  @param {string} css CSS to write to file.