Roll Dart SDK from c839eea7f0fa to af7860c1b2a5 (1 revision) (flutter/engine#44893)

https://dart.googlesource.com/sdk.git/+log/c839eea7f0fa..af7860c1b2a5

2023-08-21 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com Version 3.2.0-91.0.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://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

DEPS

@@ -53,7 +53,7 @@ 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': 'c839eea7f0fadec3232aaf7c5fe69c646bc33afb',
+  'dart_revision': 'af7860c1b2a5690817875b68419fe539fac77831',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py