[flutter_releases] Flutter beta 3.15.0-15.1.pre Engine Cherrypicks (#45831)

# Flutter beta 3.15.0-15.1.pre Engine

## Scheduled Cherrypicks

- Roll dart revision: dart-lang/sdk@4d7636806

Author: itsjustkevin

DEPS

@@ -57,7 +57,7 @@ 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': '43d4b1373788fb0a11e7700127b10f4bae1148c1',
+  'dart_revision': '4d7636806fcd58a266973da52d39385fdefb254c',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py