Roll Dart SDK from 6d06476bae6b to a9e67d81941a (3 revisions) (flutter#19677)

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': '6d06476bae6b0d1c40f6b3c9e0ef874ec3b631b7',
+  'dart_revision': 'a9e67d81941a085ba6b8d0a2fe959c47e5a8b3a2',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py