Make new StreamController be async by default.

tests/lib/async/stream_single_to_multi_subscriber_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test the basic StreamController and StreamController.singleSubscription.
 library stream_single_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:isolate';
 import '../../../pkg/unittest/lib/unittest.dart';
 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)));
     multi.listen(expectAsync1((v) => Expect.equals(42, v)));
     c.add(42);
   });
 
   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);
     Events actual2 = new Events.capture(multi);
     actual1.onDone(expectAsync0(() {
       Expect.listEquals(expected.events, actual1.events);
     }));
     actual2.onDone(expectAsync0(() {
       Expect.listEquals(expected.events, actual2.events);
     }));
     expected.replay(c);
   });
 
   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);
     Events actual2 = new Events.capture(multi);
     actual1.onDone(expectAsync0(() {
       Expect.listEquals(expected.events, actual1.events);
     }));
     actual2.onDone(expectAsync0(() {
       Expect.listEquals(expected.events, actual2.events);
     }));
     expected.replay(c);
   });
 }