Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 64b7499d7962a25f0b85cf721737264c3ca1aa5c..d0feb613b6b974872273d8409336f0e9500185ea 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -245,6 +245,8 @@ class Heap {
     old_space_.SetupImagePage(pointer, size, is_executable);
   }
 
+  bool IsDebugCollection() { return debug_gc_in_progress_; }
+
  private:
   class GCStats : public ValueObject {
    public:
@@ -346,6 +348,9 @@ class Heap {
   bool gc_new_space_in_progress_;
   bool gc_old_space_in_progress_;
 
+  // Debug GC is in progress
+  bool debug_gc_in_progress_;
+
   friend class Become;       // VisitObjectPointers
   friend class Precompiler;  // VisitObjects
   friend class ObjectGraph;  // VisitObjects