If you're sick of syntastic taking forever to check your dart files, make sure you have vim-dispatch (https://github.com/tpope/vim-dispatch) installed and stick this in your vimrc. Doesn't help the startup time of dartanalyzer, but at least your main thread won't block.

.vimrc

" dart setup
autocmd FileType dart set errorformat+=%.%#\\\|%.%#\\\|%.%#\\\|%f\\\|%l\\\|%c\\\|%.%#\\\|%m
autocmd FileType dart set makeprg=dartanalyzer\ --machine\ %
autocmd BufWritePre *.dart Make
let g:syntastic_mode_map = { "mode": "active",
      \ "passive_filetypes": ["dart"] }