Roll Dart SDK from 8531f7c2bdae to 59be21f25f2d (4 revisions) (#183547)

https://dart.googlesource.com/sdk.git/+log/8531f7c2bdae..59be21f25f2d

2026-03-11 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com
Version 3.12.0-239.0.dev
2026-03-11 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com
Version 3.12.0-238.0.dev
2026-03-11 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com
Version 3.12.0-237.0.dev
2026-03-11 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com
Version 3.12.0-236.0.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
Please CC dart-vm-team@google.com,jimgraham@google.com on the revert to
ensure that a human
is aware of the problem.

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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
diff --git a/DEPS b/DEPS
index 6e54092..488a809 100644
--- a/DEPS
+++ b/DEPS
@@ -59,7 +59,7 @@
   # updated revision list of existing dependencies. You will need to
   # gclient sync before and after update deps to ensure all deps are updated.
   # updated revision list of existing dependencies.
-  'dart_revision': '8531f7c2bdae6b18dfe439d59094ffc45db15679',
+  'dart_revision': '59be21f25f2d85174391b4d73309e178c77d942b',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py