Make new StreamController be async by default.

tests/lib/async/stream_subscription_as_future_test.dart

 // Copyright (c) 2013, 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 '../../../pkg/unittest/lib/unittest.dart';
 
 main() {
   test("subscription.asStream success", () {
     Stream stream = new Stream.fromIterable([1, 2, 3]);
     var output = [];
     var subscription = stream.listen((x) { output.add(x); });
     subscription.asFuture(output).then(expectAsync1((o) {
       Expect.listEquals([1, 2, 3], o);
     }));
   });
 
   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;
     var output = [];
     var subscription = stream.listen((x) { output.add(x); });
     subscription.asFuture(output).then(expectAsync1((o) {
       Expect.listEquals([1, 2, 3], o);
     }));
   });
 
   test("subscription.asStream success 3", () {
     Stream stream = new Stream.fromIterable([1, 2, 3]).map((x) => x);
     var output = [];
     var subscription = stream.listen((x) { output.add(x); });
     subscription.asFuture(output).then(expectAsync1((o) {
       Expect.listEquals([1, 2, 3], o);
     }));
   });
 
   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();
     Stream stream = controller.stream;
     var output = [];
     var subscription = stream.listen((x) { output.add(x); });
     subscription.asFuture(output).catchError(expectAsync1((error) {
       Expect.equals(error, "foo");
     }));
   });
 
   test("subscription.asStream failure2", () {
     Stream stream = new Stream.fromIterable([1, 2, 3, 4])
       .map((x) {
         if (x == 4) throw "foo";
         return x;
       });
     var output = [];
     var subscription = stream.listen((x) { output.add(x); });
     subscription.asFuture(output).catchError(expectAsync1((error) {
       Expect.equals(error, "foo");
     }));
   });
 }