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finally实现

Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};

1. finally返回1个Promise:
2. P.resolve(callback());调用finnally的回调，创建1个新Promise, 该Promise返回value