add flutter_driver to the allow list

Change-Id: I747999ae50a9f87a1e55bfc2f289ca2ff868f529
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165871
Auto-Submit: Jake Macdonald <jakemac@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

Author: jakemac53

pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart

@@ -166,6 +166,9 @@ const AllowedExperimentalFlags defaultAllowedExperimentalFlags =
   "flutter": {
     ExperimentalFlag.nonNullable,
   },
+  "flutter_driver": {
+    ExperimentalFlag.nonNullable,
+  },
   "flutter_test": {
     ExperimentalFlag.nonNullable,
   },

sdk/lib/_internal/allowed_experiments.json

@@ -58,6 +58,9 @@
     "flutter": {
       "experimentSet": "nullSafety"
     },
+    "flutter_driver": {
+      "experimentSet": "nullSafety"
+    },
     "flutter_test": {
       "experimentSet": "nullSafety"
     },