Roll Dart SDK from 6e17953f4e7e to ba343ec30c69 (1 revision) (flutter#28807)

https://dart.googlesource.com/sdk.git/+log/6e17953f4e7e..ba343ec30c69

2021-09-22 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.15.0-138.0.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,garyq@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

Author: skia-flutter-autoroll

DEPS

@@ -39,7 +39,7 @@ 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': '6e17953f4e7e3ce2909a0bf97e5f6c8d0c186398',
+  'dart_revision': 'ba343ec30c693133524dad1cc696a4f1d74845e0',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py