Add assert that the root widget has been attached. (#32437)

https://dart.googlesource.com/external/github.com/flutter/flutter/+/069303d46fee7cc995c2ebcf253f7c5a87ab9533
diff --git a/commits.json b/commits.json
index 8fb2c9e..c0a7c95 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"760fea25cf41aa2541f29a3123eabacd92a5a204",
-  "external/github.com/flutter/flutter":"40300fb8a905fc1a8c93cbc4148dbb9bed1e8607",
+  "external/github.com/flutter/flutter":"069303d46fee7cc995c2ebcf253f7c5a87ab9533",
   "sdk":"2fd1021326d7f2c6da68017f90e6e6523c615e05"
 }
\ No newline at end of file