Roll Dart SDK from 56fa015 to c9b47ad (1 revision) (#24408)

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': '56fa015ca3ecadadb81222c1f691c16e01ffc9ab',
+  'dart_revision': 'c9b47adb178e9107fa8b88a212f173e8c4507954',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py