Invalid dartdoc references should have visual distinction

Currently, if there is an invalid dartdoc reference (e.g. [NonExistentClass]) in IntelliJ, the only way to know that it is invalid is to try and command/ctrl-click on the reference and have it say "Cannot find declaration to go to". It would be super helpful to have those references that are invalid shown in a way that distinguishes them (and configurable in prefs). This would help in avoiding invalid links/typos in the documentation.