Add MediaQueryData.navigationMode and allow controls to be focused when disabled. (#54919)

This adds a new navigationMode to the MediaQueryData that indicates how focusable controls should behave under different navigation modes, currently with two modes: NavigationMode.traditional, and NavigationMode.directional.

It may seem like focusing a disabled control is not desirable, but this is useful for user interfaces that use DPAD navigation because if a control gets disabled, losing focus is disruptive to the user, and it is difficult to control where the focus will end up unless it is done explicitly.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/8ef291545330f18177d797a3e0ecdc5508a5eefd
diff --git a/commits.json b/commits.json
index 2f0074e..4054681 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"af14d2adf0e0933703cdf885ff2361a8f3eef00c",
-  "external/github.com/flutter/flutter":"bd06749edccc9c65151e4ce25c79e9db099fd37c",
+  "external/github.com/flutter/flutter":"8ef291545330f18177d797a3e0ecdc5508a5eefd",
   "sdk":"c703e05689125b0c17aba826cfc7e127e3adfa88"
 }
\ No newline at end of file