[vm] Reland the flag flip behind the --nnbd build flag

Bug: #40409
Bug: #40179
Change-Id: Ib84926ef15068d23dcac7b89843c454b8446b28e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134062
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Liam Appelbe <[email protected]>

Author: liamappelbe

runtime/BUILD.gn

@@ -140,6 +140,10 @@ config("dart_config") {
     }
   }
 
+  if (use_nnbd) {
+    defines += [ "DART_BUILT_WITH_NNBD_FLAG" ]
+  }
+
   if (!is_win) {
     cflags = [
       "-Werror",

runtime/vm/dart.h

@@ -123,8 +123,16 @@ 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() {
+#ifdef DART_BUILT_WITH_NNBD_FLAG
+    return true;
+#else
+    return non_nullable_flag_;
+#endif
+  }
 
  private:
   static void WaitForIsolateShutdown();