Update

autoload/dart.vim

@@ -152,4 +152,13 @@ function! s:DotPackagesFile() abort
   return [v:false, '']
 endfunction
 
+" Prevent writes to files in the pub cache.
+function! dart#setModifiable() abort
+  let full_path = expand('%:p')
+  if full_path =~# '.pub-cache' ||
+      \ full_path =~# 'Pub\Cache'
+    setlocal nomodifiable
+  endif
+endfunction
+
 endif