Relax zone allocation assertion

runtime/vm/zone.h

 // 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.
 
 #ifndef VM_ZONE_H_
 #define VM_ZONE_H_
 
 #include "platform/utils.h"
 #include "vm/allocation.h"
 #include "vm/handles.h"
@@ skipping 52 matching lines @@
 
   // Compute the total size of this zone. This includes wasted space that is
   // due to internal fragmentation in the segments.
   intptr_t SizeInBytes() const;
 
   // Structure for managing handles allocation.
   VMHandles* handles() { return &handles_; }
 
   void VisitObjectPointers(ObjectPointerVisitor* visitor);
 
+  Zone* previous() const { return previous_; }
+
  private:
   Zone();
   ~Zone();  // Delete all memory associated with the zone.
 
   // All pointers returned from AllocateUnsafe() and New() have this alignment.
   static const intptr_t kAlignment = kDoubleSize;
 
   // Default initial chunk size.
   static const intptr_t kInitialChunkSize = 1 * KB;
 
@@ skipping 168 matching lines @@
     memmove(reinterpret_cast<void*>(new_data),
             reinterpret_cast<void*>(old_data),
             old_len * kElementSize);
   }
   return new_data;
 }
 
 }  // namespace dart
 
 #endif  // VM_ZONE_H_