Process weak reference sets when a scavenge invokes the API callbacks.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index ce60bcc809d195ba0b7e8ba647b6c8153d6ce575..9eb3c4ba2f12c876cab0bcce19bf61705b143505 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -78,12 +78,15 @@ class Scavenger {
   void IterateRoots(Isolate* isolate,
                     ObjectPointerVisitor* visitor,
                     bool visit_prologue_weak_persistent_handles);
+  void IterateWeakReferences(Isolate* isolate, ObjectPointerVisitor* visitor);
   void IterateWeakRoots(Isolate* isolate,
                         HandleVisitor* visitor,
                         bool visit_prologue_weak_persistent_handles);
   void ProcessToSpace(ObjectPointerVisitor* visitor);
   void Epilogue(Isolate* isolate, bool invoke_api_callbacks);
 
+  bool IsUnreachable(RawObject** p);
+
   // During a scavenge we need to remember the promoted objects.
   // This is implemented as a stack of objects at the end of the to space. As
   // object sizes are always greater than sizeof(uword) and promoted objects do