When you move your @angular/cli version to 6.0.8 then you will face another issue while adding ng add @angular/material ------- to add both the latest Angular Material and PWA into your project Please read below.
This is just the turnaround after reading many slackoverflow and github solution.
I was successfully able to add angular material when my @angular/cli was at version 6.1.2 -->
then to add Pwa I downgrade the @angular/cli version to 6.0.8 and after I successfully added PWA again I upgraded the @angular/cli to next version v6.2.0-beta.1 then I upgraded my angular/core and angular/material.
My cli code-
**
> npm install --save @angular/cli@6.1.2
> ng new project
> ng add @angular/material
> npm install --save-dev @angular/cli@6.0.8
> ng add @angular/pwa --project <project_name>
> npm install --save @angular/cli@next
> ng update @angular/core
> ng update @angular/material
**
Hope this workaround works for you.
My Version at the moment
@angular/cli - 6.2.0-beta.1
@angular/material & @angular/cdk - 6.4.3
@angular/pwa - 0.6.8