[flutter_tool] Remove some async file io (#38654)

https://dart.googlesource.com/external/github.com/flutter/flutter/+/398ac1f6c892bc3638c08f1aef0bfbd99d8418c3
diff --git a/commits.json b/commits.json
index dba66ca..9581bf9 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"a6cd477f543a863fc44803cf7db2e39a19ba97dd",
-  "external/github.com/flutter/flutter":"28bedb10dac5cefa8e63bac76e483220644fad54",
+  "external/github.com/flutter/flutter":"398ac1f6c892bc3638c08f1aef0bfbd99d8418c3",
   "sdk":"72923471c4717821255c055574b05db05ee4cafd"
 }
\ No newline at end of file