Roll Dart SDK from 07a9404fa583 to 15b38cc239f8 (1 revision) (#31017)

diff --git a/DEPS b/DEPS
index ef685fe..b3be6ea 100644
--- a/DEPS
+++ b/DEPS
@@ -39,7 +39,7 @@
   # 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': '07a9404fa583ea8c5a4349b3d4ba8bfc852b24c7',
+  'dart_revision': '15b38cc239f842200836202823bb0bbab0cae257',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py