Roll Dart SDK from c696ecf5a8a0 to bb9d96ffbafa (1 revision) (flutter#26370)

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': 'c696ecf5a8a0ec23000aaadc2d81d77d799a4fd7',
+  'dart_revision': 'bb9d96ffbafad2cb29d57376f1cdbba79049c640',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py