code: function asCallback(promise, cb) {
    promise.then((res) => cb(null, res), cb);
  }
docs: Calls the specified callback with the result of the promise.
  
  Note that if the callback throws a synchronous error, then we will trigger an unhandled
  rejection.
  
  @param {Promise} promise
  @param {Function<Err, Any>} cb - An errback.