Implement weak persistent handles in the Dart API.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 6aee9f81e784d89053d8fe47e22a8d406fcab1ef..34e1ecd0dc073a47b1c8eb1836c1589f245e5a29 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -70,6 +70,7 @@ class Scavenger {
   uword FirstObjectStart() const { return to_->start() | object_alignment_; }
   void Prologue();
   void IterateRoots(Isolate* isolate, ObjectPointerVisitor* visitor);
+  void IterateWeakRoots(Isolate* isolate, ObjectPointerVisitor* visitor);
   void ProcessToSpace(ObjectPointerVisitor* visitor);
   void Epilogue();
 
@@ -93,6 +94,7 @@ class Scavenger {
   bool scavenging_;
 
   friend class ScavengerVisitor;
+  friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };