fix(lsp): attach dartls to the buffer that requested it

lua/flutter-tools/executable.lua

@@ -174,13 +174,13 @@ function M.get(callback)
 
   if config.flutter_lookup_cmd then
     return path_from_lookup_cmd(config.flutter_lookup_cmd, function(paths)
-      paths = {
+      cached_paths = {
         flutter_bin = paths.flutter_bin,
         flutter_sdk = paths.flutter_sdk,
         dart_bin = paths.dart_bin,
         dart_sdk = dart_sdk_root(paths),
       }
-      callback(paths)
+      callback(cached_paths)
     end)
   end
 

lua/flutter-tools/lsp/init.lua

@@ -287,12 +287,15 @@ function M.attach()
   if not is_valid_path(buffer_path) then return end
 
   get_server_config(user_config, function(c)
+    -- This callback can run asynchronously, by which point the user may have
+    -- switched to or deleted the buffer we were asked to attach to.
+    if not api.nvim_buf_is_valid(buf) then return end
     c.root_dir = M.get_project_root_dir()
       or fs.dirname(fs.find(conf.root_patterns, {
         path = buffer_path,
         upward = true,
       })[1])
-    vim.lsp.start(c)
+    vim.lsp.start(c, { bufnr = buf })
   end)
 end