[vm] Set non_nullable_flag() to true

We'll leave the deletion and cleanup of this function until later, so
that this CL is as easy to rollback as possible.

Bug: https://github.com/dart-lang/sdk/issues/40179
Change-Id: Ie583aaabd117ae06ea27a539c3bc904240a0e6bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133429
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>

runtime/vm/dart.h

@@ -123,8 +123,10 @@ class Dart : public AllStatic {
   static Dart_EntropySource entropy_source_callback() {
     return entropy_source_callback_;
   }
+
+  // TODO(dartbug.com/40342): Delete these functions.
   static void set_non_nullable_flag(bool value) { non_nullable_flag_ = value; }
-  static bool non_nullable_flag() { return non_nullable_flag_; }
+  static bool non_nullable_flag() { return true; }
 
  private:
   static void WaitForIsolateShutdown();