covariant usage has caused subtle null safety bugs when the subtype is nonnull. Example:

When null is passed into stopApp, but the override doesn't handle a null ApplicationPackage it caused runtime crash:

stderr: [   +7 ms] NoSuchMethodError: The getter 'id' was called on null.
stderr:            Receiver: null
stderr:            Tried calling: id

https://fusion2.corp.google.com/invocations/68e930bd-fdd0-486f-9b8d-dec7798677c9/targets/%2F%2Fmobile%2Fflutter%2Ftests%2Fapp:app_basic_runner_attach_ios_test_IPHONE_11_13_0/log

Audit covariant usage and remove where possible.

See https://dart.dev/guides/language/sound-problems#the-covariant-keyword