Roll Dart SDK from 331595776540 to 5b21f8a7d5d3 (1 revision) (flutter#178861)

https://dart.googlesource.com/sdk.git/+log/331595776540..5b21f8a7d5d3

2025-11-20 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com
Version 3.11.0-156.0.dev

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Author: engine-flutter-autoroll

DEPS

@@ -57,7 +57,7 @@ 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': '33159577654074c8b33c5cf0217bbcce7de4aa8e',
+  'dart_revision': '5b21f8a7d5d3d0afd8db2ca757453fdbd2b86854',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py