Roll Flutter from 28acccb388a5 to 23b9d4f1f2f2 (1 revision) (#1394)

diff --git a/.ci/flutter_master.version b/.ci/flutter_master.version
index 88f6074..c8c598c 100644
--- a/.ci/flutter_master.version
+++ b/.ci/flutter_master.version
@@ -1 +1 @@
-28acccb388a5445054c25fac415b1d4cb0b0b338
+23b9d4f1f2f2ba86d273089f52668f90694f3e03