Set VMTag from stubs when transitioning between Dart and Native

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 5410ab82d7c7f5c87ac8161d5ea1f9d043913d18..24ed4f57631a4135252ceb799bf7285bec95470e 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -176,6 +176,10 @@ class Isolate : public BaseIsolate {
     return OFFSET_OF(Isolate, top_exit_frame_info_);
   }
 
+  static intptr_t vm_tag_offset() {
+    return OFFSET_OF(Isolate, vm_tag_);
+  }
+
   ApiState* api_state() const { return api_state_; }
   void set_api_state(ApiState* value) { api_state_ = value; }