VM: Fix crash in profile signal handler.

runtime/vm/thread_registry.cc

Index: runtime/vm/thread_registry.cc
diff --git a/runtime/vm/thread_registry.cc b/runtime/vm/thread_registry.cc
index 264b844f453d716f9a4653a07b4807ae44baad2d..0d36e0d2575cac44dc1d0474baf0cbe140db0bc8 100644
--- a/runtime/vm/thread_registry.cc
+++ b/runtime/vm/thread_registry.cc
@@ -106,11 +106,11 @@ void ThreadRegistry::Unschedule(Thread* thread,
   OSThread* os_thread = thread->os_thread();
   ASSERT(os_thread != NULL);
   os_thread->DisableThreadInterrupts();
+  os_thread->set_thread(NULL);
+  OSThread::SetCurrent(os_thread);
   thread->isolate_ = NULL;
   thread->heap_ = NULL;
   thread->set_os_thread(NULL);
-  os_thread->set_thread(NULL);
-  OSThread::SetCurrent(os_thread);
   if (!is_mutator) {
     ASSERT(thread->api_top_scope() == NULL);
     ReturnThreadToFreelist(thread);