feat: add dart capabilities & refactor server cmd

Lets us track which capabilities the current dart version supports,
which will help for version regressions that may be introduced or for
deciding what features we can use.

Author: elken

lsp-dart.el

@@ -110,11 +110,25 @@ If unspecified, diagnostics will not be generated."
         (append (list (file-name-directory (buffer-file-name))) lsp-dart-extra-library-directories)
       lsp-dart-extra-library-directories)))
 
+(defun lsp-dart--dart-capabiliities (&optional sdk-root)
+  "Given an SDK-ROOT, return capabilities for that dart version.
+Default to `lsp-dart-get-sdk-dir' if no root is passed.
+Returns nil if an invalid SDK-ROOT has been passed."
+  (when-let* ((root (or sdk-root lsp-dart-sdk-dir (lsp-dart-get-sdk-dir)))
+              (dart-version (string-trim
+                             (with-temp-buffer
+                               (insert-file-contents (expand-file-name "version" root))
+                               (buffer-string)))))
+    `((canUseLanguageServer . ,(version<= "3.14.4" dart-version)))))
+
 (defun lsp-dart--server-command ()
   "Generate LSP startup command."
   (or lsp-dart-server-command
       (list (lsp-dart-dart-command)
-            "language-server"
+            (if (alist-get 'canUseLanguageServer (lsp-dart--dart-capabiliities))
+                "language-server"
+              (expand-file-name "bin/snapshots/analysis_server.dart.snapshot" (lsp-dart-get-sdk-dir))
+              "--lsp")
             "--client-id emacs.lsp-dart"
             (format "--client-version %s" lsp-dart-version-string))))