Roll Dart SDK from 9bc9e48fe908 to b191fb71063c (1 revision) (flutter#24847)

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': '9bc9e48fe90883562836a91e072d5309902f577e',
+  'dart_revision': 'b191fb71063c3141612cc12fdf7c50c6f4a2b958',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py