Roll Dart SDK from 874709e52a99 to a3d902d8598e (1 revision) (flutter#22026)

https://dart.googlesource.com/sdk.git/+log/874709e52a99..a3d902d8598e

2020-10-21 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.11.0-242.0.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

Author: skia-flutter-autoroll

DEPS

@@ -34,7 +34,7 @@ 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': '874709e52a9955999ae5634a166341a333a92843',
+  'dart_revision': 'a3d902d8598eabf68db42c6f7911fb7fb63f7012',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py