code: function processFallback(originUrl, dir, options) {
      if (typeof options.fallback === 'function') {
          return options.fallback.apply(null, arguments);
      }
      switch (options.fallback) {
          case 'copy':
              return processCopy.apply(null, arguments);
          case 'rebase':
              return processRebase.apply(null, arguments);
          default:
              return Promise.resolve();
      }
  }
docs: @param {String} originUrl
  @param {PostcssUrl~Dir} dir
  @param {PostcssUrl~Option} options
  
  @returns {String|Undefined}