Reinstate unsat for sub dynamic

Summary:
There is some special 'postprocessing' of error messages under sound-dynamic which we are currently ignoring under extended reasons.

This diff makes the behavior under extended reasons consistent with the legacy case to ensure we have no regression in error messages.

Reviewed By: madgen

Differential Revision: D63455212

fbshipit-source-id: ce3722e3316ad6f34e351770c6da6a4986eef859

Author: Michael Thomas

hphp/hack/src/typing/typing_subtype.ml

@@ -4070,21 +4070,17 @@ end = struct
       and ty_name = lazy (Pretty.describe_ty_default env (LoclType ty_sub))
       and pos = Reason.to_pos (get_reason ty_sub) in
       let postprocess =
-        if TypecheckerOptions.using_extended_reasons env.genv.tcopt then
-          fun prop ->
-        prop
-        else
-          if_unsat
-            (invalid
-               ~fail:
-                 (Option.map
-                    subtype_env.Subtype_env.on_error
-                    ~f:
-                      Typing_error.(
-                        fun on_error ->
-                          apply_reasons ~on_error
-                          @@ Secondary.Not_sub_dynamic
-                               { pos; ty_name; dynamic_part })))
+        if_unsat
+          (invalid
+             ~fail:
+               (Option.map
+                  subtype_env.Subtype_env.on_error
+                  ~f:
+                    Typing_error.(
+                      fun on_error ->
+                        apply_reasons ~on_error
+                        @@ Secondary.Not_sub_dynamic
+                             { pos; ty_name; dynamic_part })))
       in
       postprocess
       @@