对于Angular 8到9的更新，我是按照官方文档进行升级的。
这建议首先更新到最新版本的 Angular 8，例如：
ng update @angular/core@8 @angular/cli@8
但是，我收到三个警告（全部相同）：
npm WARN notsup Unsupported engine for watchpack-chokidar2@2.0.0: wanted: {"node":"<8.10.0"} (current: {"node":"12.16.1","npm":"6.13.4"})
npm WARN notsup Not compatible with your version of node/npm: watchpack-chokidar2@2.0.0
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@~2.1.2 (node_modules\watchpack\node_modules\chokidar\node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@2.1.3: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})
npm WARN ngx-multi-window@0.3.2 requires a peer of @angular/common@^6.0.0-rc.0 || ^6.0.0 || ^7.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN ngx-multi-window@0.3.2 requires …