Clean VM thread shutdown:

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index 3b3eb3308d30b292147168449f670ba1f62033f5..9345b645a8644effe83be23ea6f75bb05bd72b0b 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -134,11 +134,8 @@ void Thread::EnterIsolate(Isolate* isolate) {
   ASSERT(thread->thread_state() == NULL);
   InterruptableThreadState* thread_state =
       ThreadInterrupter::GetCurrentThreadState();
-#if defined(DEBUG)
-  thread->set_thread_state(NULL);  // Exclude thread itself from the dupe check.
-  Isolate::CheckForDuplicateThreadState(thread_state);
-  thread->set_thread_state(thread_state);
-#endif
+  // TODO(koda): Calling Isolate::CheckForDuplicateThreadState(thread_state)
+  // here can lead to deadlock. Evaluate doing this check some other way.
   ASSERT(thread_state != NULL);
   // TODO(koda): Migrate profiler interface to use Thread.
   Profiler::BeginExecution(isolate);