Warning when a registerFallbackValue is never used

[alestiago]

Description
As a user I would like to get a warning whenever a fallback value has been registered but has never been used.

void main() {
    setUpAll(() {
      registerFallbackValue(Foo()); // Should result in a warning since it is never used.
    });
}

Possible solutions

• Runtime warning
• Custom analyser lint rule

Describe alternatives you've considered
N/A

Additional context

• https://invertase.io/blog/announcing-dart-custom-lint