Fix race and limit access to mutator_thread_.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index a79b086b7e5c0b587049a5e48c17b71d8c18d9ff..aa4f67ea92a647b00488f466a0e6f9ed26af02e4 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -123,9 +123,9 @@ void Thread::EnterIsolate(Isolate* isolate) {
   Thread* thread = Thread::Current();
   ASSERT(thread != NULL);
   ASSERT(thread->isolate() == NULL);
-  ASSERT(isolate->mutator_thread() == NULL);
+  ASSERT(!isolate->HasMutatorThread());
   thread->isolate_ = isolate;
-  isolate->set_mutator_thread(thread);
+  isolate->MakeCurrentThreadMutator(thread);
   // TODO(koda): Migrate thread_state_ and profile_data_ to Thread, to allow
   // helper threads concurrent with mutator.
   ASSERT(isolate->thread_state() == NULL);
@@ -162,7 +162,7 @@ void Thread::ExitIsolate() {
   }
   isolate->set_thread_state(NULL);
   Profiler::EndExecution(isolate);
-  isolate->set_mutator_thread(NULL);
+  isolate->ClearMutatorThread();
   thread->isolate_ = NULL;
   ASSERT(Isolate::Current() == NULL);
   thread->heap_ = NULL;
@@ -178,7 +178,7 @@ void Thread::EnterIsolateAsHelper(Isolate* isolate) {
   thread->heap_ = isolate->heap();
   // Do not update isolate->mutator_thread, but perform sanity check:
   // this thread should not be both the main mutator and helper.
-  ASSERT(isolate->mutator_thread() != thread);
+  ASSERT(!isolate->MutatorThreadIsCurrentThread());
   thread->Schedule(isolate);
 }
 
@@ -193,7 +193,7 @@ void Thread::ExitIsolateAsHelper() {
   thread->Unschedule();
   thread->isolate_ = NULL;
   thread->heap_ = NULL;
-  ASSERT(isolate->mutator_thread() != thread);
+  ASSERT(!isolate->MutatorThreadIsCurrentThread());
 }