Roll Dart SDK from 8bab67d to 21aa7f8 (1 revision) (flutter#25829)

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': '8bab67dad50b08782e24d7b851f34cfbe5a9ef24',
+  'dart_revision': '21aa7f8cbde83eb1d1b6bc9ec87b9ef5a19a76b3',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py