A nodejs question, really need some help here

[顾培]

 * Implement the below function, which takes a node stream, writes the output to

 * the file in 'path', and returns a promise which resolves when the stream is

 * exhausted and all data is successfully written to the file system.

 *

 * To write to the file system, use `fs-promise`, which provides promise wrappers

 * around things like `fs.writeFile()`. Do _not_ use `fs.createWriteStream()`.

 *

 * @param {Stream} stream a streams3-compatible stream, not in object mode

 * @param {String} filePath

 * @return {Promise} resolves when all data successfully written, rejects if any error

 */

function writeStreamToFile(stream, filePath)