popper.js项目组不提供下载，理由如下：
https://popper.js.org/docs/v2/
3. Direct Download?
Managing dependencies by “directly downloading” them and placing them into your source code is not recommended for a variety of reasons, including missing out on feat/fix updates easily. Please use a versioning management system like a CDN or npm/Yarn.
可以通过https://unpkg.com/browse/[email protected]/ 下载
如图：