Add cast to prepare for package:file update (#33268)

https://dart.googlesource.com/external/github.com/flutter/flutter/+/c6129b4e85c596a93b0c253352d0bbdfcf04695d
diff --git a/commits.json b/commits.json
index 59c2a65..35a9fa9 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"01b8c078cb00f1b853c01a95405ca125b2114b1e",
-  "external/github.com/flutter/flutter":"bc7bc940836f1f834699625426795fd6f07c18ec",
+  "external/github.com/flutter/flutter":"c6129b4e85c596a93b0c253352d0bbdfcf04695d",
   "sdk":"6e1f628f19c1ed0ccd2b12a86d09a78ad34767bb"
 }
\ No newline at end of file