Fix race and limit access to mutator_thread_.

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "vm/code_observers.h"
@@ skipping 598 matching lines @@
 
 
 #if defined(DEBUG)
 // static
 void BaseIsolate::AssertCurrent(BaseIsolate* isolate) {
   ASSERT(isolate == Isolate::Current());
 }
 
 void BaseIsolate::AssertCurrentThreadIsMutator() const {
   ASSERT(Isolate::Current() == this);
-  ASSERT(Isolate::Current()->mutator_thread() == Thread::Current());
+  ASSERT(Isolate::Current()->MutatorThreadIsCurrentThread());
 }
 #endif  // defined(DEBUG)
 
 #if defined(DEBUG)
 #define REUSABLE_HANDLE_SCOPE_INIT(object)                                     \
   reusable_##object##_handle_scope_active_(false),
 #else
 #define REUSABLE_HANDLE_SCOPE_INIT(object)
 #endif  // defined(DEBUG)
 
@@ skipping 1539 matching lines @@
                            serialized_message_, serialized_message_len_);
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(I);
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart