[flutter_tools] cache the base URL as index.html (#53666)

https://dart.googlesource.com/external/github.com/flutter/flutter/+/bb5c3400669aed84585b92f67a422d21a42da0e2
diff --git a/commits.json b/commits.json
index 9626b06..7c7f8b8 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"f7c6c6d92a126e5ad2a45bd28c6aa1ee1195a281",
-  "external/github.com/flutter/flutter":"d62c7ecce54a9a386afe036460b553c77826aecd",
+  "external/github.com/flutter/flutter":"bb5c3400669aed84585b92f67a422d21a42da0e2",
   "sdk":"43079207d4b6e5b7954b29cc43747bd7a6595176"
 }
\ No newline at end of file