Implement a 2-pass heap verification algorithm.

runtime/vm/visitor.h

Index: runtime/vm/visitor.h
diff --git a/runtime/vm/visitor.h b/runtime/vm/visitor.h
index cdd0c5bd03e0aa5c56116999f1fddd3e71ac5c58..20b48b38e2811b0c436c706e0436e353153de5bd 100644
--- a/runtime/vm/visitor.h
+++ b/runtime/vm/visitor.h
@@ -41,10 +41,19 @@ class ObjectPointerVisitor {
 // An object visitor interface.
 class ObjectVisitor {
  public:
+  explicit ObjectVisitor(Isolate* isolate) : isolate_(isolate) {}
+
   virtual ~ObjectVisitor() {}
 
+  Isolate* isolate() const { return isolate_; }
+
   // Invoked for each object.
   virtual void VisitObject(RawObject* obj) = 0;
+
+ private:
+  Isolate* isolate_;
+
+  DISALLOW_COPY_AND_ASSIGN(ObjectVisitor);
 };