Fix race condition with HasMutatorThread that was being used without a lock.

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 VM_THREAD_REGISTRY_H_
 #define VM_THREAD_REGISTRY_H_
 
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
 #include "vm/stack_frame.h"
 #include "vm/thread.h"
 
 namespace dart {
 
 // Unordered collection of threads relating to a particular isolate.
 class ThreadRegistry {
  public:
   ThreadRegistry()
       : threads_lock_(new Monitor()),
         active_list_(NULL),
         free_list_(NULL),
         mutator_thread_(NULL) {}
   ~ThreadRegistry();
 
   void VisitObjectPointers(ObjectPointerVisitor* visitor, bool validate_frames);
   void PrepareForGC();
+  Thread* mutator_thread() const { return mutator_thread_; }
 
  private:
   Thread* active_list() const { return active_list_; }
   Monitor* threads_lock() const { return threads_lock_; }
 
   Thread* GetFreeThreadLocked(Isolate* isolate, bool is_mutator);
   void ReturnThreadLocked(bool is_mutator, Thread* thread);
   void AddToActiveListLocked(Thread* thread);
   void RemoveFromActiveListLocked(Thread* thread);
   Thread* GetFromFreelistLocked(Isolate* isolate);
@@ skipping 21 matching lines @@
   Thread* mutator_thread_;
 
   friend class Isolate;
   friend class SafepointHandler;
   DISALLOW_COPY_AND_ASSIGN(ThreadRegistry);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_REGISTRY_H_