build(@angular/pwa): promote to stable
build(@angular/pwa): promote to stable

As discussed in the weekly sync meeting, promote `@angular/pwa` to stable
since it is just a schematic library and does not have public facing APIs.

Brocco/angular-cli,catull/angular-cli,ocombe/angular-cli,Brocco/angular-cli,clydin/angular-cli,ocombe/angular-cli,clydin/angular-cli,catull/angular-cli,geofffilippi/angular-cli,angular/angular-cli,angular/angular-cli,ocombe/angular-cli,Brocco/angular-cli,catull/angular-cli,Brocco/angular-cli,catull/angular-cli,clydin/angular-cli,ocombe/angular-cli,geofffilippi/angular-cli,angular/angular-cli,clydin/angular-cli,angular/angular-cli,ocombe/angular-cli,Brocco/angular-cli,geofffilippi/angular-cli,geofffilippi/angular-cli