Make new StreamController be async by default.

tests/standalone/io/io_sink_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.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 class TestConsumer implements StreamConsumer {
@@ skipping 47 matching lines @@
   sink = new IOSink(new TestConsumer([0, 1, 2]));
   sink.add([0]);
   sink.add([1]);
   sink.add([2]);
   sink.close();
 }
 
 void testAddStreamClose() {
   {
     var sink = new IOSink(new TestConsumer([0]));
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     sink.addStream(controller.stream)
         .then((_) {
           sink.close();
         });
     controller.add([0]);
     controller.close();
   }
   {
     var sink = new IOSink(new TestConsumer([0, 1, 2]));
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     sink.addStream(controller.stream)
         .then((_) {
           sink.close();
         });
     controller.add([0]);
     controller.add([1]);
     controller.add([2]);
     controller.close();
   }
 }
 
 void testAddStreamAddClose() {
   {
     var sink = new IOSink(new TestConsumer([0, 1]));
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     sink.addStream(controller.stream)
         .then((_) {
           sink.add([1]);
           sink.close();
         });
     controller.add([0]);
     controller.close();
   }
 }
 
 void main() {
   testClose();
   testAddClose();
   testAddStreamClose();
   testAddStreamAddClose();
 }