Assert helper threads do not (yet) attempt to access the "current zone".

runtime/vm/base_isolate.h

Index: runtime/vm/base_isolate.h
diff --git a/runtime/vm/base_isolate.h b/runtime/vm/base_isolate.h
index 067c70882a30580ffc2221fe36eae773ed3588a7..dccb44b4c38d51f90912d39f7df568be2afc0a93 100644
--- a/runtime/vm/base_isolate.h
+++ b/runtime/vm/base_isolate.h
@@ -26,8 +26,16 @@ class BaseIsolate {
   }
 
   // DEPRECATED: Use Thread::current_zone.
-  Zone* current_zone() const { return current_zone_; }
+  Zone* current_zone() const {
+    AssertCurrentThreadIsMutator();
+    return current_zone_;
+  }
   void set_current_zone(Zone* zone) { current_zone_ = zone; }
+#if defined(DEBUG)
+  void AssertCurrentThreadIsMutator() const;
+#else
+  void AssertCurrentThreadIsMutator() const {}
+#endif  // DEBUG
 
   HandleScope* top_handle_scope() const {
 #if defined(DEBUG)