Make new StreamController be async by default.

tests/standalone/io/mime_multipart_parser_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:math';
 import 'dart:io';
 import 'dart:isolate';
 
 void testParse(String message,
                String boundary,
                [List<Map> expectedHeaders,
                List expectedParts,
                bool expectError = false]) {
   void testWrite(List<int> data, [int chunkSize = -1]) {
-    StreamController controller = new StreamController();
+    StreamController controller = new StreamController(sync: true);
 
     var stream = controller.stream.transform(
         new MimeMultipartTransformer(boundary));
     int i = 0;
     var port = new ReceivePort();
     stream.listen((multipart) {
       int part = i++;
       if (expectedHeaders != null) {
         Expect.mapEquals(expectedHeaders[part], multipart.headers);
       }
@@ skipping 281 matching lines @@
 \r
 Body2\r
 --xxx\r\n""";
   testParse(message, "xxx", null, [null, null], true);
 }
 
 void main() {
   testParseValid();
   testParseInvalid();
 }