Use a shorter doc header for CustomMatcher (dart-lang/matcher#256)

Separate the second sentence into a new paragraph so dartdoc does not
pull it in to the header. Rephrase the header to say more than that it
is a base class.

Update language version to `3.4` to match the test runner.

Remove some unnecessary cast and `!` from private field promotion.
7 files changed
tree: 4bd9f50cc0678ddb4f3e50f1ab88afe5bc3d11cb
  1. pkgs/