Roll src/third_party/dart 5f9e3e07915b..15ee305516d7 (4 commits) (#18498)

diff --git a/DEPS b/DEPS
index fe2d1dd..4069be5 100644
--- a/DEPS
+++ b/DEPS
@@ -34,7 +34,7 @@
   # 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': '5f9e3e07915b90a99867f1acd6dd9d3ac9bc9049',
+  'dart_revision': '15ee305516d76925dcb3d19463e82a998b074957',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py