Roll Dart SDK from 855e2764c295 to a8b6687327d6 (1 revision) (flutter/engine#41817)

https://dart.googlesource.com/sdk.git/+log/855e2764c295..a8b6687327d6

2023-05-08 dart-internal-merge@dart-ci-internal.iam.gserviceaccount.com Version 3.1.0-86.0.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 aam@google.com,dart-vm-team@google.com,jimgraham@google.com,rmistry@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': '855e2764c2958ca0208c218b3298a3e0f5713f52',
+  'dart_revision': 'a8b6687327d66fb77f427319fa2220b00f37286b',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py