Roll Dart SDK from a9d5833 to d257741 (1 revision) (#22258)

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': 'a9d5833834109773eb0186242d3dd9a778807d83',
+  'dart_revision': 'd2577410a5016eadc111919355e19d42dd45d816',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py