dead links for some libraries on master branch API docs

[jcollins-g]

https://master-docs-flutter-io.firebaseapp.com/

See entries on the left under "Dart Core" that have no hyperlink.

Dartdoc v0.18.1 will correct this.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.