node_modules/@angular/common/locales/package.json

{
  "description": "This directory contains UMD files which by its nature are not side-effect free. The parent package.json marks the whole @angular/common package as side-effect free, so if this directory is not excluded from that, build-optimizer will mark the contents as side-effect free and uglify will incorrectly remove all needed localization code.",
  "sideEffects": true
}