eglot keeps sending didSave notifications despite the active server not supporting textDocumentSync

[6b6279]

Steps to reproduce issue:

1. Create example Dart project using Flutter: flutter create hello_world
2. Open hello_world/lib/main.dart in Emacs
3. Start eglot with M-x eglot RET
4. Make a rudimentary change in the file; ex. add a line break
5. Save with C-x C-s
   Following text appears in the minibuffer area: [eglot] Server reports (type=1): Unknown method textDocument/didSave

See dart-lang/sdk#36464 and microsoft/language-server-protocol#288

Thanks in advance.

[6b6279]

Filed a bug report on the official GNU bug tracker: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=61478

[joaotavora]

👍 make sure to cc me in a message to that bug, so i can join the discussion (and potentially do the patch)