Make new StreamController be async by default.

tests/lib/async/stream_single_to_multi_subscriber_test.dart

Index: tests/lib/async/stream_single_to_multi_subscriber_test.dart
diff --git a/tests/lib/async/stream_single_to_multi_subscriber_test.dart b/tests/lib/async/stream_single_to_multi_subscriber_test.dart
index 1f4d6934db9decf5a9550daea58c31cd29061316..8fd37e13b0c5d984135394924df1199059eda0f9 100644
--- a/tests/lib/async/stream_single_to_multi_subscriber_test.dart
+++ b/tests/lib/async/stream_single_to_multi_subscriber_test.dart
@@ -13,7 +13,7 @@ import 'event_helper.dart';
 
 main() {
   test("tomulti 1", () {
-    StreamController c = new StreamController<int>();
+    StreamController c = new StreamController<int>(sync: true);
     Stream<int> multi = c.stream.asBroadcastStream();
     // Listen twice.
     multi.listen(expectAsync1((v) => Expect.equals(42, v)));
@@ -22,7 +22,7 @@ main() {
   });
 
   test("tomulti 2", () {
-    StreamController c = new StreamController<int>();
+    StreamController c = new StreamController<int>(sync: true);
     Stream<int> multi = c.stream.asBroadcastStream();
     Events expected = new Events.fromIterable([1, 2, 3, 4, 5]);
     Events actual1 = new Events.capture(multi);
@@ -37,7 +37,7 @@ main() {
   });
 
   test("tomulti no-op", () {
-    StreamController c = new StreamController<int>();
+    StreamController c = new StreamController<int>(sync: true);
     Stream<int> multi = c.stream.asBroadcastStream();
     Events expected = new Events.fromIterable([1, 2, 3, 4, 5]);
     Events actual1 = new Events.capture(multi);