code: async function buildSassFiles(files, outputDir, config) {
    const renderedFiles = await Promise.all(files.map(async file => {
      const css = await renderFile(file, config);
      return {file, css};
    }));
  
    await fs.ensureDir(outputDir);
  
    return Promise.all(renderedFiles.map(({file, css}) => {
      const outputFile = getOutputFile(file, outputDir);
      return writeFile(outputFile, css);
    }));
  }
docs: Builds sass files to output dir
  @param {!Array.<string>} files
  @param {!string} outputDir
  @param {!Object} config
  @return {!Promise}