Make new StreamController be async by default.

tests/lib/async/stream_single_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 Stream.single method.
 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("single", () {
-    StreamController c = new StreamController();
+    StreamController c = new StreamController(sync: true);
     Future f = c.stream.single;
     f.then(expectAsync1((v) { Expect.equals(42, v);}));
     new Events.fromIterable([42]).replay(c);
   });
 
   test("single empty", () {
-    StreamController c = new StreamController();
+    StreamController c = new StreamController(sync: true);
     Future f = c.stream.single;
     f.catchError(expectAsync1((error) { Expect.isTrue(error is StateError); }));
     new Events.fromIterable([]).replay(c);
   });
 
   test("single error", () {
-    StreamController c = new StreamController();
+    StreamController c = new StreamController(sync: true);
     Future f = c.stream.single;
     f.catchError(expectAsync1((error) { Expect.equals("error", error); }));
     Events errorEvents = new Events()..error("error")..close();
     errorEvents.replay(c);
   });
 
   test("single error 2", () {
-    StreamController c = new StreamController();
+    StreamController c = new StreamController(sync: true);
     Future f = c.stream.single;
     f.catchError(expectAsync1((error) { Expect.equals("error", error); }));
     Events errorEvents = new Events()..error("error")..error("error2")..close();
     errorEvents.replay(c);
   });
 
   test("single error 3", () {
-    StreamController c = new StreamController();
+    StreamController c = new StreamController(sync: true);
     Future f = c.stream.single;
     f.catchError(expectAsync1((error) { Expect.equals("error", error); }));
     Events errorEvents = new Events()..add(499)..error("error")..close();
     errorEvents.replay(c);
   });
 }