Clean VM thread shutdown:

tests/isolate/nested_spawn_test.dart

Index: tests/isolate/nested_spawn_test.dart
diff --git a/tests/isolate/nested_spawn_test.dart b/tests/isolate/nested_spawn_test.dart
index 0dc35a07bc86f266218a701358083b73b9182977..083a94a9aa454343a9a4167c619725d7a30ef247 100644
--- a/tests/isolate/nested_spawn_test.dart
+++ b/tests/isolate/nested_spawn_test.dart
@@ -24,7 +24,7 @@ void main([args, port]) {
   test("spawned isolates can spawn nested isolates", () {
     ReceivePort port = new ReceivePort();
     Isolate.spawn(isolateA, [port.sendPort, "main"]);
-    port.first.then((message) {
+    return port.first.then((message) {
       expect("main", message[1]);
       expect("isolateA", message[2]);
       expect("isolateB", message[3]);