Roll Dart SDK from 42976b2 to 9cb9c6d (1 revision) (#31067)

Author: skia-flutter-autoroll

DEPS

@@ -39,7 +39,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/main/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '42976b23388651d913bb7182f4d43ab73db4627c',
+  'dart_revision': '9cb9c6d71976ee68c37124aac34da4c1b59ec01c',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py