Make new StreamController be async by default.

tests/lib/async/stream_subscription_as_future_test.dart

Index: tests/lib/async/stream_subscription_as_future_test.dart
diff --git a/tests/lib/async/stream_subscription_as_future_test.dart b/tests/lib/async/stream_subscription_as_future_test.dart
index 5be8f7951bec3e20b2d7aad41df63b648df8433c..3c2c082ddeffe0045ab3788ef292e7a84fad847f 100644
--- a/tests/lib/async/stream_subscription_as_future_test.dart
+++ b/tests/lib/async/stream_subscription_as_future_test.dart
@@ -20,7 +20,7 @@ main() {
   });
 
   test("subscription.asStream success2", () {
-    StreamController controller = new StreamController();
+    StreamController controller = new StreamController(sync: true);
     [1, 2, 3].forEach(controller.add);
     controller.close();
     Stream stream = controller.stream;
@@ -41,7 +41,7 @@ main() {
   });
 
   test("subscription.asStream failure", () {
-    StreamController controller = new StreamController();
+    StreamController controller = new StreamController(sync: true);
     [1, 2, 3].forEach(controller.add);
     controller.addError("foo");
     controller.close();