regression: warning for _TrivialWebGoldenComparator in flutter

[jcollins-g]

Comparing proposed version 0.29.3 to 0.29.2 results in a new warning:

*** Flutter repo : 1 new warnings in HEAD, missing in v0.29.2
  warning: unresolved doc reference [_TrivialWebGoldenComparator]
    from flutter_test.webGoldenComparator: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/fluttert2PXVD/packages/flutter_test/lib/src/goldens.dart:240:25)
    in documentation inherited from goldens.webGoldenComparator: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/fluttert2PXVD/packages/flutter_test/lib/src/goldens.dart:240:25)
*** Flutter repo : Difference in warning output found for 1 warnings (1226 warnings found)"

We should not be losing track of the private class this references. Does not change the doc output (we still can't link to a private), but Dartdoc is not supposed to warn on valid private references.

[srawlins]

Stale.