dartdoc_options error settings are escaping containment via inherited documentation

[jcollins-g]

As in typeahead 0.1.5, if documentation is inherited from Flutter and is unable to resolve a documentation comment due to other bugs in dartdoc (like #2389), it will flag the package inheriting it with an error.

[jcollins-g]

example:

  error: unresolved doc reference [DiagnosticsTreeStyle]
from typeahead.material.TypeaheadBaseState.debugFillProperties:
../…/widgets/framework.dart:1474
in documentation inherited from diagnostics.Diagnosticable.debugFillProperties: