Make StreamSubscription be the active part of a stream.

tests/standalone/io/http_multipart_test.dart

 // Copyright (c) 2012, 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 98 matching lines @@
 
 
   postDataTest(message.codeUnits,
                'multipart/form-data',
                'AaB03x',
                [new FormField('submit-name', 'Larry'),
                 new FormField('files',
                               'Content of file',
                               contentType: 'text/plain',
                               filename: 'file1.txt')]);
-
   // Similar test using Chrome posting.
   message = [
       45, 45, 45, 45, 45, 45, 87, 101, 98, 75, 105, 116, 70, 111, 114, 109, 66,
       111, 117, 110, 100, 97, 114, 121, 81, 83, 113, 108, 56, 107, 68, 65, 76,
       77, 55, 116, 65, 107, 67, 49, 13, 10, 67, 111, 110, 116, 101, 110, 116,
       45, 68, 105, 115, 112, 111, 115, 105, 116, 105, 111, 110, 58, 32, 102,
       111, 114, 109, 45, 100, 97, 116, 97, 59, 32, 110, 97, 109, 101, 61, 34,
       115, 117, 98, 109, 105, 116, 45, 110, 97, 109, 101, 34, 13, 10, 13, 10,
       84, 101, 115, 116, 13, 10, 45, 45, 45, 45, 45, 45, 87, 101, 98, 75, 105,
       116, 70, 111, 114, 109, 66, 111, 117, 110, 100, 97, 114, 121, 81, 83, 113,
@@ skipping 62 matching lines @@
   postDataTest(message,
                'multipart/form-data',
                '----WebKitFormBoundaryfe0EzV1aNysD1bPh',
                [new FormField('name', 'øv')]);
 }
 
 
 void main() {
   testPostData();
 }