Prepares allocation for proper sync with mutator and bg threads.

runtime/vm/thread_registry.h

 // Copyright (c) 2015, 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 RUNTIME_VM_THREAD_REGISTRY_H_
 #define RUNTIME_VM_THREAD_REGISTRY_H_
 
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
@@ skipping 61 matching lines @@
   // on the same thread.
   // We probably need a mechanism to return to the specific thread only
   // for these specific cases. We should also determine if the embedder
   // should allow exiting an isolate with live state in zones/handles in
   // which case a new API for returning to the specific thread needs to be
   // added.
   Thread* mutator_thread_;
 
   friend class Isolate;
   friend class SafepointHandler;
+  friend class Scavenger;
   DISALLOW_COPY_AND_ASSIGN(ThreadRegistry);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_THREAD_REGISTRY_H_