Fix timing issue in stream_empty_test, and make analyzer expect it to pass.

tests/lib/async/stream_empty_test.dart

Index: tests/lib/async/stream_empty_test.dart
diff --git a/tests/lib/async/stream_empty_test.dart b/tests/lib/async/stream_empty_test.dart
index 133ea48b25ed6d99eb663936d1cb35d5257446fa..c1d4b7b34f82c365201fe4fefbdcded36a0a5b0f 100644
--- a/tests/lib/async/stream_empty_test.dart
+++ b/tests/lib/async/stream_empty_test.dart
@@ -7,7 +7,12 @@ import "package:expect/expect.dart";
 import "dart:async";
 import 'package:async_helper/async_helper.dart';
 
-main() async {
+main() {
+  asyncStart();
+  runTest().whenComplete(asyncEnd);
+}
+
+Future runTest() async {
   unreachable([a,b]) { throw "UNREACHABLE"; }
   int tick = 0;
   ticker() { tick++; }