Enable iterating over roots from helper threads by moving stack walking.

runtime/vm/thread.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 VM_THREAD_H_
 #define VM_THREAD_H_
 
 #include "vm/globals.h"
 #include "vm/os_thread.h"
 #include "vm/store_buffer.h"
@@ skipping 244 matching lines @@
   }
 
   static void SetCurrent(Thread* current);
 
   void Schedule(Isolate* isolate);
   void Unschedule();
 
   friend class ApiZone;
   friend class Isolate;
   friend class StackZone;
+  friend class ThreadRegistry;
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_