Moving @MirrorsUsed out of angular.dart

The issue is that types added via @MirrorsUsed can never be removed, so these cannot be dropped if users use the static generators.

This moves the @MirrorsUsed into an optional import, which means that all of the samples need to be updated to explicitly include this.

The benefit is that we can then migrate over to the static generators without having a large breaking change.

I also added explicit overrides so the override: '*' is not needed.