Roll Dart SDK from ff7e92c7886d to b21443096387 (1 revision) (flutter/engine#49199)

https://dart.googlesource.com/sdk.git/+log/ff7e92c7886d..b21443096387

2023-12-18 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com Version 3.3.0-241.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC dart-vm-team@google.com,jsimmons@google.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Flutter Engine: https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

DEPS

@@ -66,7 +66,7 @@ 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': 'ff7e92c7886dc4d78ccf21f870e620725811b31b',
+  'dart_revision': 'b21443096387b281a14e947b300df3e9f91054f2',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

engine/src/flutter/ci/licenses_golden/licenses_dart

@@ -1,4 +1,4 @@
-Signature: d99bcb9a47be134ddce44fd02437b94c
+Signature: 06486b08d62f2d74956d6d88fab85d66
 
 ====================================================================================================
 LIBRARY: dart