Avoid race in isolate shutdown; add assertions, error messages

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index a82c8b3a358dafbde915c9522441ccdfeb670f32..bbdd1b401859582a0217cf7ffae509277f385b0c 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -27,6 +27,12 @@ static void DeleteThread(void* thread) {
 }
 
 
+Thread::~Thread() {
+  // We should cleanly exit any isolate before destruction.
+  ASSERT(isolate_ == NULL);
+}
+
+
 void Thread::InitOnceBeforeIsolate() {
   ASSERT(thread_key_ == OSThread::kUnsetThreadLocalKey);
   thread_key_ = OSThread::CreateThreadLocal(DeleteThread);