Roll Dart SDK from eb24e3324908 to 99970d646dc9 (1 revision) (flutter#21239)

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': 'eb24e3324908a0987881a05d64c7e8d45ead9c4a',
+  'dart_revision': '99970d646dc9d7895c5f633150ec75022b711501',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py