Clean up violations of the "prefer_is_not_operator" Dart lint.
Clean up violations of the "prefer_is_not_operator" Dart lint.

Lint documentation: https://dart-lang.github.io/linter/lints/prefer_is_not_operator
Discussion: http://g/dart-lints/I0cvlcUqvTI
LSC: http://go/dart-trivial-lint-cleanup-lsc
PiperOrigin-RevId: 361155269

dart-lang/mockito