`unnecessary_non_null_assertion` for `isNull`/`isNotNull` matcher

[FMorschel]

I'm proposing we have something like unnecessary_non_null_assertion (or even use this diagnostic) for the cases where the isNotNull/isNull matcher is being used, but makes no sense. Probably a leftover from some old API that is not null anymore.

Of course, the isNull tests would fail, but this would help users migrating the tests to the new api.

CC @bwilkerson @srawlins @stereotype441 @natebosch

[srawlins]

I think this would not be a very high value lint rule to implement (in an existing rule or as a new one), and would be better implemented in an analyzer plugin for the test package. I've implemented this rule in this open PR: dart-lang/test#2461

[FMorschel]

Alright then, thanks!