Fix race and limit access to mutator_thread_.

runtime/vm/thread_registry.cc

 // 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.
 
 #include "vm/thread_registry.h"
 
 #include "vm/isolate.h"
 #include "vm/lockers.h"
 
 namespace dart {
 
 void ThreadRegistry::SafepointThreads() {
   MonitorLocker ml(monitor_);
   // First wait for any older rounds that are still in progress.
   while (in_rendezvous_) {
     // Assert we are not the organizer trying to nest calls to SafepointThreads.
     ASSERT(remaining_ > 0);
     CheckSafepointLocked();
   }
   // Start a new round.
   in_rendezvous_ = true;
   ++round_;  // Overflows after 240+ years @ 10^9 safepoints per second.
   remaining_ = CountScheduledLocked();
   Isolate* isolate = Isolate::Current();
   // We only expect this method to be called from within the isolate itself.
   ASSERT(isolate->thread_registry() == this);
   // TODO(koda): Rename Thread::PrepareForGC and call it here?
   --remaining_;  // Exclude this thread from the count.
   // Ensure the main mutator will reach a safepoint (could be running Dart).
-  if (Thread::Current() != isolate->mutator_thread()) {
+  if (!isolate->MutatorThreadIsCurrentThread()) {
     isolate->ScheduleInterrupts(Isolate::kVMInterrupt);
   }
   while (remaining_ > 0) {
     ml.Wait(Monitor::kNoTimeout);
   }
 }
 
 
 void ThreadRegistry::ResumeAllThreads() {
   MonitorLocker ml(monitor_);
@@ skipping 34 matching lines @@
   for (int i = 0; i < entries_.length(); ++i) {
     const Entry& entry = entries_[i];
     if (entry.scheduled) {
       ++count;
     }
   }
   return count;
 }
 
 }  // namespace dart