old_contents: 'use strict'
  
  const sass = require('sass')
  
  /**
   * Transform SASS to CSS.
   * @public
   * @param {String} folderPath - Path to the folder containing the SASS file.
   * @param {String} str - SASS.
   * @param {Object} opts - Optional options for the task.
   * @returns {Promise<String>} CSS.
   */
  module.exports = async function(folderPath, str, opts) {
  
  	// SASS can't handle empty files
  	if (str === '') return str
  
  	// Dismiss sourceMap when output should be optimized
  	const sourceMap = opts.optimize !== true
  
  	const result = sass.renderSync({
  		data: str,
  		includePaths: [ folderPath ],
  		sourceMap: sourceMap === true ? '' : false,
  		sourceMapEmbed: sourceMap,
  		sourceMapContents: sourceMap
  	})
  
  	return result.css.toString()
  
  }
new_contents: 'use strict'
  
  const util = require('util')
  const sass = require('sass')
  
  /**
   * Transform SASS to CSS.
   * @public
   * @param {String} folderPath - Path to the folder containing the SASS file.
   * @param {String} str - SASS.
   * @param {Object} opts - Optional options for the task.
   * @returns {Promise<String>} CSS.
   */
  module.exports = async function(folderPath, str, opts) {
  
  	// SASS can't handle empty files
  	if (str === '') return str
  
  	// Dismiss sourceMap when output should be optimized
  	const sourceMap = opts.optimize !== true
  
  	const result = await util.promisify(sass.render)({
  		data: str,
  		includePaths: [ folderPath ],
  		sourceMap: sourceMap === true ? '' : false,
  		sourceMapEmbed: sourceMap,
  		sourceMapContents: sourceMap
  	})
  
  	return result.css.toString()
  
  }
subject: Switch to async version