Move reusable handles from isolate to thread.

runtime/vm/thread_registry.h

Index: runtime/vm/thread_registry.h
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index cf32f24f66e73ac88b787a3cce8cafc5608de01f..daa3cdd2b8d3f7314fc855958c86cf6d50cfd2ba 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -129,6 +129,8 @@ class ThreadRegistry {
       if (state.zone != NULL) {
         state.zone->VisitObjectPointers(visitor);
       }
+      ASSERT(entry.thread != NULL);
+      entry.thread->VisitObjectPointers(visitor);
       // Iterate over all the stack frames and visit objects on the stack.
       StackFrameIterator frames_iterator(state.top_exit_frame_info,
                                          validate_frames);