Roll Flutter from 99fb2d36e807 to a4b246f2a88c (28 revisions) (#6530)

diff --git a/.ci/flutter_master.version b/.ci/flutter_master.version
index 52837a9..df69827 100644
--- a/.ci/flutter_master.version
+++ b/.ci/flutter_master.version
@@ -1 +1 @@
-99fb2d36e80737c16b51b3e014fc996c901fdabe
+a4b246f2a88c3f5b838c5abff8063a4e7436d44b