[cfe] Implement support for dynamic checks on parameters that are covariant-by-declaration due to inheritance

[eernstg]

This is the front end specific issue for #47072, which has the details.

Note that the front end may already have the new behavior (it already succeeds in a run of the tests added by https://dart-review.googlesource.com/c/sdk/+/208504).

[devoncarew]

@johnniwinther - can you provide a very rough estimate of how long this would take to do? We're trying to get breadbox estimates of the ~5 impl. tasks for this language change.

[johnniwinther]

The CFE part test 1-2 days (more). I'm working on it now.

[johnniwinther]

A rough cornercase turned up. I'm working on a fix, but we can release this feature without it.

[johnniwinther]

Fix by https://dart-review.googlesource.com/c/sdk/+/216420