Fix --verify-on-transition for OSR and concurrent sweep.

runtime/vm/lockers.cc

Index: runtime/vm/lockers.cc
diff --git a/runtime/vm/lockers.cc b/runtime/vm/lockers.cc
index b8ac4b64c84f27ce4180e057526fd77f9ac2d8fc..39733dac483604d048046bb2595f3a0505b704e6 100644
--- a/runtime/vm/lockers.cc
+++ b/runtime/vm/lockers.cc
@@ -29,6 +29,7 @@ static void updateThreadState(Thread* thread) {
 Monitor::WaitResult MonitorLocker::WaitWithSafepointCheck(Thread* thread,
                                                           int64_t millis) {
   ASSERT(thread == Thread::Current());
+  ASSERT(thread->execution_state() == Thread::kThreadInVM);
   thread->set_execution_state(Thread::kThreadInBlockedState);
   thread->EnterSafepoint();
   Monitor::WaitResult result = monitor_->Wait(millis);