Implement weak persistent handles in the Dart API.

runtime/vm/handles.h

Index: runtime/vm/handles.h
diff --git a/runtime/vm/handles.h b/runtime/vm/handles.h
index acd8153a2a0a486bd051438aaeb55eea6e936e8b..12dbed9c03238ee2b731aff75c9bc3dde964636e 100644
--- a/runtime/vm/handles.h
+++ b/runtime/vm/handles.h
@@ -58,6 +58,15 @@ namespace dart {
 class ObjectPointerVisitor;
 
 
+class HandleVisitor {
+ public:
+  virtual void Visit(uword* addr) = 0;
+
+  virtual ~HandleVisitor() {
+  }
+};
+
+
 template <int kHandleSizeInWords, int kHandlesPerChunk, int kOffsetOfRawPtr>
 class Handles {
  public:
@@ -73,6 +82,9 @@ class Handles {
   // Visit all object pointers stored in the various handles.
   void VisitObjectPointers(ObjectPointerVisitor* visitor);
 
+  // Visits all of the various handles.
+  void Visit(HandleVisitor* visitor);
+
   // Allocates a handle in the current handle scope. This handle is valid only
   // in the current handle scope and is destroyed when the current handle
   // scope ends.
@@ -142,6 +154,9 @@ class Handles {
     // Visit all object pointers in the handle block.
     void VisitObjectPointers(ObjectPointerVisitor* visitor);
 
+    // Visit all of the handles in the handle block.
+    void Visit(HandleVisitor* visitor);
+
 #if defined(DEBUG)
     // Zaps the free handle area to an uninitialized value.
     void ZapFreeHandles();