code: function writeFile(file, css) {
    return new Promise((resolve, reject) => {
      fs.writeFile(file, css, err => {
        if (err) {
          return reject(err);
        }
        resolve();
      })
    });
  }
docs: Writes css to file
  @param {!string} file
  @param {!string} css
  @return {!Promise}