[vm, gc] Require a safepoint for heap iteration.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 62572410ee3a6bd156e7fd0fd7705c346fbb7e55..fea025f5bf56228017711d1fa195e5b1c3069ba8 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -178,12 +178,6 @@ class Isolate : public BaseIsolate {
   void RegisterClassAt(intptr_t index, const Class& cls);
   void ValidateClassTable();
 
-  // Visit all object pointers.
-  void IterateObjectPointers(ObjectPointerVisitor* visitor,
-                             bool validate_frames);
-  void IterateStackPointers(ObjectPointerVisitor* visitor,
-                            bool validate_frames);
-
   // Visits weak object pointers.
   void VisitWeakPersistentHandles(HandleVisitor* visitor);
 
@@ -929,8 +923,9 @@ class Isolate : public BaseIsolate {
   friend class Become;    // VisitObjectPointers
   friend class GCMarker;  // VisitObjectPointers
   friend class SafepointHandler;
-  friend class Scavenger;    // VisitObjectPointers
   friend class ObjectGraph;  // VisitObjectPointers
+  friend class Scavenger;    // VisitObjectPointers
+  friend class HeapIterationScope;  // VisitObjectPointers
   friend class ServiceIsolate;
   friend class Thread;
   friend class Timeline;