Reverts VM thread cleanup

runtime/vm/thread.cc

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