[Wildcard Variables] Dartdoc Support

[kallentu]

This issue will track the work needed in Dartdoc for the wildcard variables feature.

There's probably no additional work needed for Dartdoc.
However, we should test that Dartdoc isn't broken by changes to how we represent wildcard variables in the analyzer.

[kallentu]

cc @srawlins

This issue tracks if we need anything for Dartdoc. We probably won't, but we can cross that bridge later when we have the analyzer work done :)

[bwilkerson]

If Dartdoc is still doing its own resolution of identifier references in doc comments, then a reference of the form [_] will need to be resolved differently. From dartdoc's perspective, this feature will only effect parameters, and that might be the only place that will be impacted by the change.

[srawlins]

From dartdoc's perspective, this feature will only effect parameters, and that might be the only place that will be impacted by the change.

I think also type parameters, import prefixes, and maybe record type positional fields. In particular field formal parameters and super parameters should be tested.