Revert "[vm] Set non_nullable_flag() to true"

This reverts commit 61b4bbc.

Reason for revert: Performance regressions

Original change's description:
> [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: #40179
> Change-Id: Ie583aaabd117ae06ea27a539c3bc904240a0e6bc
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133429
> Reviewed-by: Siva Annamalai <[email protected]>
> Commit-Queue: Liam Appelbe <[email protected]>

[email protected],[email protected]

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: #40179
Change-Id: I7e1d81286d769faeacec7dc17612ff18d157f114
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134060
Reviewed-by: Liam Appelbe <[email protected]>
Commit-Queue: Liam Appelbe <[email protected]>

Author: liamappelbe

runtime/vm/dart.h

@@ -123,10 +123,8 @@ 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 true; }
+  static bool non_nullable_flag() { return non_nullable_flag_; }
 
  private:
   static void WaitForIsolateShutdown();