code: function contentsAsync(file, options, cb) {
    if (typeof options === 'function') {
      cb = options;
      options = {};
    }
  
    if (typeof cb !== 'function') {
      throw new TypeError('expected a callback function');
    }
  
    if (!utils.isObject(file)) {
      cb(new TypeError('expected file to be an object'));
      return;
    }
  
    contentsSync(file, options);
    cb(null, file);
  }
docs: Async method for getting `file.contents`.
  
  @param  {Object} `file`
  @param  {Object} `options`
  @param  {Function} `cb`
  @return {Object}