Make new StreamController be async by default.

tests/standalone/io/http_server_response_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
@@ skipping 84 matching lines @@
         .then((response) {
           request.response.addStream(new File(new Options().script).openRead())
               .then((response) {
                 response.close();
                 response.done.then((_) => server.close());
               });
         });
   }, bytes: bytes * 2);
 
   testServerRequest((server, request) {
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     request.response.addStream(controller.stream)
         .then((response) {
           response.close();
           response.done.then((_) => server.close());
         });
     controller.close();
   }, bytes: 0);
 
   testServerRequest((server, request) {
     request.response.addStream(new File("__not_exitsing_file_").openRead())
@@ skipping 157 matching lines @@
 
 void main() {
   testResponseDone();
   testResponseAddStream();
   testResponseAddStreamClosed();
   testResponseAddClosed();
   testBadResponseAdd();
   testBadResponseClose();
   testIgnoreRequestData();
 }