Roll Dart SDK from df54886 to 996a581 (1 revision) (#23450)

Author: skia-flutter-autoroll

DEPS

@@ -34,7 +34,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': 'df54886d12953bab553b90e257d3844dd72c7165',
+  'dart_revision': '996a581228215f7db426a18c2f003078138b87cb',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py