Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 8d8c0e49e3fa121840f8d51ba958548d3c5401b9..fd4663de7decf7680a9cd26dc3581d2af389e295 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -149,11 +149,7 @@ class Scavenger {
   void Scavenge(bool invoke_api_callbacks);
 
   // Promote all live objects.
-  void Evacuate() {
-    Scavenge();
-    Scavenge();
-    ASSERT(UsedInWords() == 0);
-  }
+  void Evacuate();
 
   // Accessors to generate code for inlined allocation.
   uword* TopAddress() { return &top_; }
@@ -297,6 +293,8 @@ class Scavenger {
   // The total size of external data associated with objects in this scavenger.
   intptr_t external_size_;
 
+  bool failed_to_promote_;
+
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;