Language version bumps for publish_to: none pkgs

Also a separate pubspec for the tools/ folder

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

Change-Id: If49a6ede07e4864d0c0fcb210c04c18c804849df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/412041
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

pkg/js_shared/pubspec.yaml

@@ -3,7 +3,7 @@ name: js_shared
 publish_to: none
 
 environment:
-  sdk: ^3.0.0
+  sdk: ^3.5.0
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dev_dependencies: