Fix race and limit access to mutator_thread_.

runtime/vm/dart_api_impl.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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "vm/class_finalizer.h"
@@ skipping 1424 matching lines @@
   DARTSCOPE(isolate);
   return Api::NewHandle(isolate, String::New(isolate->name()));
 }
 
 
 
 DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate) {
   CHECK_NO_ISOLATE(Isolate::Current());
   // TODO(16615): Validate isolate parameter.
   Isolate* iso = reinterpret_cast<Isolate*>(isolate);
-  if (iso->mutator_thread() != NULL) {
+  if (iso->HasMutatorThread()) {
     FATAL("Multiple mutators within one isolate is not supported.");
   }
   Thread::EnsureInit();
   Thread::EnterIsolate(iso);
 }
 
 
 DART_EXPORT void Dart_IsolateBlocked() {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
@@ skipping 4404 matching lines @@
   ASSERT(stream != NULL);
   TimelineEvent* event = stream->StartEvent();
   if (event != NULL) {
     event->AsyncEnd(label, async_id);
     event->Complete();
   }
   return Api::Success();
 }
 
 }  // namespace dart