Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 2309e6ca2aa3bb5c5b08cdf062cc4556206060b5..bbd96bde955d0be8e375f5b4b2d17c16dbbd4a73 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -896,4 +896,15 @@ void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(external_size_ >= 0);
 }
 
+void Scavenger::UnmarkNewSpace() {
+  uword cur = FirstObjectStart();
+  while (cur < top_) {
+    RawObject* raw_obj = RawObject::FromAddr(cur);
+    if (raw_obj->IsMarked() && !raw_obj->IsVMHeapObject()) {
+      raw_obj->ClearMarkBit();
+    }
+    cur += raw_obj->Size();
+  }
+}
+
 }  // namespace dart