VM: Fix an app-jit related shutdown race.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 115a3a9d6ab7a998565c936479bd94e8f3cc2357..153299f0b57185384843562f7bd5a759c00b5216 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -399,6 +399,14 @@ class Isolate : public BaseIsolate {
     return shutdown_callback_;
   }
 
+  static void SetCleanupCallback(Dart_IsolateCleanupCallback cb) {
+    cleanup_callback_ = cb;
+  }
+  static Dart_IsolateCleanupCallback CleanupCallback() {
+    return cleanup_callback_;
+  }
+
+
   void set_object_id_ring(ObjectIdRing* ring) { object_id_ring_ = ring; }
   ObjectIdRing* object_id_ring() { return object_id_ring_; }
 
@@ -851,6 +859,7 @@ class Isolate : public BaseIsolate {
 
   static Dart_IsolateCreateCallback create_callback_;
   static Dart_IsolateShutdownCallback shutdown_callback_;
+  static Dart_IsolateCleanupCallback cleanup_callback_;
 
   static void WakePauseEventHandler(Dart_Isolate isolate);