1. Provide a framework to traverse persistent prolog weak handles.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 34879)
+++ runtime/vm/isolate.cc	(working copy)
@@ -861,6 +861,13 @@
 }
 
 
+void Isolate::VisitPrologueWeakPersistentHandles(HandleVisitor* visitor) {
+  if (api_state() != NULL) {
+    api_state()->VisitPrologueWeakHandles(visitor);
+  }
+}
+
+
 void Isolate::PrintToJSONStream(JSONStream* stream, bool ref) {
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", (ref ? "@Isolate" : "Isolate"));