Roll Dart SDK from 9ae3e22f1e53 to 6d15e04622c2 (1 revision) (flutter#20483)

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': '9ae3e22f1e530e8234decd3fd5bb3498f46c26d1',
+  'dart_revision': '6d15e04622c25d9430832d25f69dcee1cdd1d494',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py