Make new StreamController be async by default.

pkg/http/test/multipart_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.
 
 library multipart_test;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:utf';
 
@@ skipping 136 matching lines @@
   });
 
   // TODO(nweiz): test creating a multipart file with a charset other than UTF-8
   // once issue 6284 is fixed.
 
   // TODO(nweiz): test creating a string with a unicode body once issue 6284 is
   // fixed.
 
   test('with a stream file', () {
     var request = new http.MultipartRequest('POST', dummyUrl);
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     request.files.add(new http.MultipartFile('file', controller.stream, 5));
 
     expect(request, bodyMatches('''
         --{{boundary}}
         content-type: application/octet-stream
         content-disposition: form-data; name="file"
 
         hello
         --{{boundary}}--
         '''));
 
     controller.add([104, 101, 108, 108, 111]);
     controller.close();
   });
 
   test('with an empty stream file', () {
     var request = new http.MultipartRequest('POST', dummyUrl);
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     request.files.add(new http.MultipartFile('file', controller.stream, 0));
 
     expect(request, bodyMatches('''
         --{{boundary}}
         content-type: application/octet-stream
         content-disposition: form-data; name="file"
 
 
         --{{boundary}}--
         '''));
@@ skipping 39 matching lines @@
         content-type: application/octet-stream
         content-disposition: form-data; name="file"; filename="test-file"
 
         hello
         --{{boundary}}--
         '''));
       }), completes);
     });
   });
 }