In combination with the TS `noImplicitOverride` compatibility changes,
we also want to follow the best-practice of adding `override` to
members which are implemented as part of abstract classes. This
commit fixes all instances which will be flagged as part of the
custom `no-implicit-override-abstract` TSLint rule.
PR Close#42512
Adds a migration that casts the value of `ActivatedRouteSnapshot.fragment` to be non-nullable.
Also moves some code from the `AbstractControl.parent` migration so that it can be reused.
Relates to #37336.
PR Close#41092