Use a tag bit for indicating prologue weak persistent handles, this avoids

runtime/vm/scavenger.cc

 // Copyright (c) 2011, 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/scavenger.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 256 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(ScavengerVisitor);
 };
 
 
 class ScavengerWeakVisitor : public HandleVisitor {
  public:
   explicit ScavengerWeakVisitor(Scavenger* scavenger) : scavenger_(scavenger) {
   }
 
-  void VisitHandle(uword addr) {
+  void VisitHandle(uword addr, bool is_prologue_weak) {
     FinalizablePersistentHandle* handle =
         reinterpret_cast<FinalizablePersistentHandle*>(addr);
     RawObject** p = handle->raw_addr();
     if (scavenger_->IsUnreachable(p)) {
-      FinalizablePersistentHandle::Finalize(handle);
+      FinalizablePersistentHandle::Finalize(handle, is_prologue_weak);
     }
   }
 
  private:
   Scavenger* scavenger_;
 
   DISALLOW_COPY_AND_ASSIGN(ScavengerWeakVisitor);
 };
 
 
@@ skipping 431 matching lines @@
   space.AddProperty("name", "Scavenger");
   space.AddProperty("user_name", "new");
   space.AddProperty("collections", collections());
   space.AddProperty("used", UsedInWords() * kWordSize);
   space.AddProperty("capacity", CapacityInWords() * kWordSize);
   space.AddProperty("time", RoundMicrosecondsToSeconds(gc_time_micros()));
 }
 
 
 }  // namespace dart