Make new StreamController be async by default.

pkg/http/lib/src/multipart_request.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.
 
 library multipart_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 import 'dart:utf';
@@ skipping 71 matching lines @@
 
   /// Freezes all mutable fields and returns a single-subscription [ByteStream]
   /// that will emit the request body.
   ByteStream finalize() {
     // TODO(nweiz): freeze fields and files
     var boundary = _boundaryString(_BOUNDARY_LENGTH);
     headers['content-type'] = 'multipart/form-data; boundary="$boundary"';
     headers['content-transfer-encoding'] = 'binary';
     super.finalize();
 
-    var controller = new StreamController<List<int>>();
+    var controller = new StreamController<List<int>>(sync: true);
 
     void writeAscii(String string) {
       assert(isPlainAscii(string));
       controller.add(string.codeUnits);
     }
 
     writeUtf8(String string) => controller.add(encodeUtf8(string));
     writeLine() => controller.add([13, 10]); // \r\n
 
     fields.forEach((name, value) {
@@ skipping 59 matching lines @@
   String _boundaryString(int length) {
     var prefix = "dart-http-boundary-";
     var list = new List<int>(length - prefix.length);
     for (var i = 0; i < list.length; i++) {
       list[i] = _BOUNDARY_CHARACTERS[
           _random.nextInt(_BOUNDARY_CHARACTERS.length)];
     }
     return "$prefix${new String.fromCharCodes(list)}";
   }
 }