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

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 34879)
+++ runtime/include/dart_api.h	(working copy)
@@ -592,6 +592,14 @@
     Dart_GcPrologueCallback prologue_callback,
     Dart_GcEpilogueCallback epilogue_callback);
 
+typedef void (*Dart_GcPrologueWeakHandleCallback)(void* isolate_callback_data,
+                                                  Dart_WeakPersistentHandle obj,
+                                                  intptr_t num_native_fields,
+                                                  intptr_t* native_fields);
+
+DART_EXPORT Dart_Handle Dart_VisitPrologueWeakHandles(
+    Dart_GcPrologueWeakHandleCallback callback);
+
 /*
  * ==========================
  * Initialization and Globals