Fix Windows event-handler, where write can 'complete' with 0 bytes written.

sdk/lib/io/http_impl.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.
 
 part of dart.io;
 
 const int _HEADERS_BUFFER_SIZE = 8 * 1024;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
@@ skipping 1325 matching lines @@
     _httpParser.responseToMethod = method;
     _streamFuture = outgoing.done
         .then((s) {
           // Request sent, set up response completer.
           _nextResponseCompleter = new Completer();
 
           // Listen for response.
           _nextResponseCompleter.future
               .then((incoming) {
                 _currentUri = null;
-                incoming.dataDone.then((_) {
-                  if (!_dispose &&
+                incoming.dataDone.then((closing) {
+                  if (!closing &&
+                      !_dispose &&
                       incoming.headers.persistentConnection &&
                       request.persistentConnection) {
                     // Return connection, now we are done.
                     _httpClient._returnConnection(this);
                     _subscription.resume();
                   } else {
                     destroy();
                   }
                 });
                 // For digest authentication if proxy check if the proxy
@@ skipping 1210 matching lines @@
   const _RedirectInfo(this.statusCode, this.method, this.location);
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }