Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/pages.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/pages.h"
 
 #include "platform/address_sanitizer.h"
 #include "platform/assert.h"
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
@@ skipping 941 matching lines @@
           prev_page = page;
         } else {
           FreePage(page, prev_page);
         }
         // Advance to the next page.
         page = next_page;
       }
 
       mid3 = OS::GetCurrentMonotonicMicros();
 
+      { heap_->new_space()->UnmarkNewSpace(); }
+
       if (!FLAG_concurrent_sweep) {
         // Sweep all regular sized pages now.
         prev_page = NULL;
         page = pages_;
         while (page != NULL) {
           HeapPage* next_page = page->next();
           bool page_in_use =
               sweeper.SweepPage(page, &freelist_[page->type()], true);
           if (page_in_use) {
             prev_page = page;
           } else {
             FreePage(page, prev_page);
           }
           // Advance to the next page.
           page = next_page;
         }
+
         if (FLAG_verify_after_gc) {
           OS::PrintErr("Verifying after sweeping...");
           heap_->VerifyGC(kForbidMarked);
           OS::PrintErr(" done.\n");
         }
       } else {
         // Start the concurrent sweeper task now.
         GCSweeper::SweepConcurrent(isolate, pages_, pages_tail_,
                                    &freelist_[HeapPage::kData]);
       }
@@ skipping 319 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result = static_cast<int>(
         (static_cast<double>(gc_time) / static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart