Roll Dart SDK from 61d949d7ec1b to b2b8c997eefa (1 revision) (#25277)

https://dart.googlesource.com/sdk.git/+log/61d949d7ec1b..b2b8c997eefa

2021-03-28 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.13.0-172.0.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 on the revert to ensure that a human
is aware of the problem.

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/master/autoroll/README.md

DEPS

@@ -35,7 +35,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '61d949d7ec1bccae40cdd25b59fa24b1b46ba714',
+  'dart_revision': 'b2b8c997eefa6e09d5ce4008d63c0c586c6d856d',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py