fix(@schematics/update): update @angular/pwa when migrating to 8

[alan-agius4]

Related to #14589 (comment)

[Splaktar]

It's good that the @angular/pwa package will now stay in sync, if it's installed. However, the package doesn't seem to provide any useful functionality other than it's ng add schematic.

• Are there plans to actually add ng g schematics for @angular/pwa?
• It seems like ng update @angular/service-worker would handle breaking changes there if any should occur?
• Is the idea that @angular/pwa would provide an update schematic if the Web App Manifest standards had some breaking changes or important new features?

I'm just trying to figure out the value of this package staying installed after ng add is run.

[clydin]

There is currently no method to execute ng add schematics without installing the containing package first. This is actually something I have considered supporting as there is the potential for a package that will only serve as a vehicle for the addition schematics and having it otherwise installed just adds noise to the project.

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