Bump language version for pkg/macro

Extracted from https://dart-review.googlesource.com/c/sdk/+/397164

I don't know if this package will still continue to exist.

Change-Id: Ifc00f1c650eb844cbf8025358710c098847f666c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/412043
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Jake Macdonald <jakemac@google.com>

pkg/macros/pubspec.yaml

@@ -6,7 +6,7 @@ description: >-
 repository: https://github.com/dart-lang/sdk/tree/main/pkg/macros
 
 environment:
-  sdk: ^3.4.0-256.0.dev
+  sdk: ^3.5.0
 
 dependencies:
   _macros: