service workers files are not minified #11538

- [ ] bug report -> please search issues before submitting
- [x] feature request

- [X] devkit
- [ ] schematics

Angular CLI: 6.0.8
Node: 8.9.0
OS: win32 x64
Angular: 6.0.9
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router, service-worker

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.6.8
@angular-devkit/build-angular     0.6.8
@angular-devkit/build-optimizer   0.6.8
@angular-devkit/core              0.6.8
@angular-devkit/schematics        0.6.8
@angular/cli                      6.0.8
@ngtools/webpack                  6.0.8
@schematics/angular               0.6.8
@schematics/update                0.6.8
rxjs                              6.2.1
typescript                        2.7.2
webpack                           4.8.3

ng  generate serviceWorker --project my-app-ngsw
ng build --prod