Roll Dart SDK from 6b7054122356 to a6fb0acd71ea (1 revision) (flutter#24631)

Author: skia-flutter-autoroll

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': '6b7054122356943abb002b8140499fc9330559dc',
+  'dart_revision': 'a6fb0acd71eaf1b4d6fae15b9a430ea6458d4195',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py