[Dot Shorthands] Dartdoc Support

[kallentu]

This issue tracks the work needed for supporting dot shorthands in Dartdoc.

Estimated Work

• Confirm that [.id] or [.id()] should not work (since there's no context available)

I don't see how else this could be used, not sure. Will add as we think of tasks.

cc. @srawlins

[srawlins]

SG. Usually these expression-level features are straightforward in dartdoc.