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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 8d0c3c12f66fb5025e95c0a039283569b5855a27..08d0ba6b39d6db27be3ba16264864bf6a6026faa 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -603,6 +603,11 @@ void Isolate::Flags::CopyTo(Dart_IsolateFlags* api_flags) const {
 void BaseIsolate::AssertCurrent(BaseIsolate* isolate) {
   ASSERT(isolate == Isolate::Current());
 }
+
+void BaseIsolate::AssertCurrentThreadIsMutator() const {
+  ASSERT(Isolate::Current() == this);
+  ASSERT(Isolate::Current()->mutator_thread() == Thread::Current());
+}
 #endif  // defined(DEBUG)
 
 #if defined(DEBUG)