[flutter_releases] Flutter beta 3.21.0-0.1.pre Engine Cherrypicks (#51271)

# Flutter beta 3.21.0-0.1.pre Engine

## Scheduled Cherrypicks

- Roll dart revision: dart-lang/sdk@c38c85a42

Author: itsjustkevin

DEPS

@@ -62,7 +62,7 @@ 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': '728fcf0bcbcac3eb5c2f1a3c2d7280d4026af897',
+  'dart_revision': 'c38c85a427ca82d80e1e07887bc2f53f5e3f8114',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py