Clear reusable handles of helper threads as well. Visit mutator_thread_ even if it is not active (a…

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index c5c6e70a7bb8524c897de04779dcfa75b19f837f..90dfaef6d0aa8a93e3d18a05537d1581e33ded52 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -262,6 +262,8 @@ void Thread::ExitIsolateAsHelper(bool bypass_safepoint) {
   ASSERT(thread != NULL);
   ASSERT(!thread->IsMutatorThread());
   ASSERT(thread->execution_state() == Thread::kThreadInVM);
+  // Clear since GC will not visit the thread once it is unscheduled.
+  thread->ClearReusableHandles();
   thread->StoreBufferRelease();
   Isolate* isolate = thread->isolate();
   ASSERT(isolate != NULL);