I just added a new plugin (vim-loves-dafny) to my ~/.config/nvim/init.vim file. I've got the necessary executables installed.
% which dafny
/usr/local/bin//dafny
But /dafny doesn't find anything in :help syntastic-checkers, and when I edit a Dafny file, I don't get any sort of highlighting.
I'm an idiot and just needed to enable the checker.
Fixed by adding
let g:syntastic_dafny_checkers = ['dafny']to my config.