Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 9c52356321181e21b265ba2db60c021b566e4d9c..6e0300e080f34d546eeced9ff4489c171151e334 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -959,6 +959,8 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks) {
 
       mid3 = OS::GetCurrentMonotonicMicros();
 
+      { heap_->new_space()->UnmarkNewSpace(); }
+
       if (!FLAG_concurrent_sweep) {
         // Sweep all regular sized pages now.
         prev_page = NULL;
@@ -975,6 +977,7 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks) {
           // Advance to the next page.
           page = next_page;
         }
+
         if (FLAG_verify_after_gc) {
           OS::PrintErr("Verifying after sweeping...");
           heap_->VerifyGC(kForbidMarked);