Automatic thread cleanup on non-Windows platforms.

runtime/vm/thread_pool.cc

Index: runtime/vm/thread_pool.cc
===================================================================
--- runtime/vm/thread_pool.cc	(revision 45815)
+++ runtime/vm/thread_pool.cc	(working copy)
@@ -274,11 +274,8 @@
 
     // Release monitor while handling the task.
     monitor_.Exit();
-    Thread::EnsureInit();
     task->Run();
     ASSERT(Isolate::Current() == NULL);
-    // Prevent unintended sharing of state between tasks.
-    Thread::CleanUp();
     delete task;
     monitor_.Enter();
 
@@ -318,6 +315,7 @@
 
 // static
 void ThreadPool::Worker::Main(uword args) {
+  Thread::EnsureInit();
   Worker* worker = reinterpret_cast<Worker*>(args);
   worker->Loop();
 
@@ -333,6 +331,9 @@
     ml.Notify();
   }
   delete worker;
+#if defined(TARGET_OS_WINDOWS)
+  Thread::CleanUp();
+#endif
 }
 
 }  // namespace dart