Here is how it looks in VS Code in the context of Dart/Flutter - the Code Actions (under the refactor menu) allow quickly adding code for Flutters "widgets":

I haven't tried the Dart LSP server (I'm on arm64 linux under macOS and haven't found the corresponding dart binaries) but in general this feature should be implemented. When you right-click something in the editor, these should be present under the Commands submenu. There's also a new keybinding you can assign to quickly pop up a menu with all the available commands.

I haven't had a chance to recompile with the latest support and try your LSP plugin

Would be useful if someone did to see if everything in geany-lsp works for Dart.