[flutter_releases] Flutter stable 3.19.6 Engine Cherrypicks (#52199)

# Flutter stable 3.19.6 Engine

## Scheduled Cherrypicks

- Roll dart revision: dart-lang/sdk@d70d99a91

Author: godofredoc

DEPS

@@ -66,7 +66,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/main/DEPS
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': 'c09cb46304325cc59890ef685d33f5e022da047e',
+  'dart_revision': 'd70d99a911b3316ca0d442caaf24fe57afe59893',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py