documentation inheritance loses lookup context for Fields

[jcollins-g]

Found in #2819.

Documentation inheritance (where dartdoc will take some documentation from a supertype for a partially-overridden Field) loses context for setter documentation, so if users refer to symbols or names unavailable in the inherited context dartdoc will emit warnings. This can be surprising for the user as it will propagate across package boundaries.

The warning can be squelched, but there is no other workaround at present.