unused_element false-positive when using super parameters on private classes

[mateusfccp]

Describe the issue
Consider the following code:

class _MyWidget extends StatelessWidget {
  const _MyWidget({Key? key}) : super(key: key);

  // ...
}

This code won't trigger any info/warning.

However, if I use super parameters instead:

class _MyWidget extends StatelessWidget {
  const _MyWidget({super.key});

  // ...
}

The following info will be issued:
info: A value for optional parameter 'key' isn't ever given. (unused_element at [...] lib/...)

To Reproduce
Use the code above with Dart 2.17+ and unused_elements enabled.

Expected behavior
The linter should behave equally for both codes, as they are virtually the same. When using super parameters, however, the linter will incorrectly fire an info.

Additional context:
This issue, obviously, won't happen if the class is public.

Possible workaround:
A possible workaround is to provide a value, like ({super.key = null}). However, it does conflict with avoid_init_to_null.

[mateusfccp]

Duplicate of dart-lang/sdk#49025.