Preserve '\' in filenames for uploads, as Windows/IE contains them.

pkg/http_server/lib/src/http_multipart_form_data_impl.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.
 
 part of http_server;
 
 
 class _HttpMultipartFormData extends Stream implements HttpMultipartFormData {
   final ContentType contentType;
   final HeaderValue contentDisposition;
@@ skipping 54 matching lines @@
       switch (key) {
         case 'content-type':
           type = ContentType.parse(multipart.headers[key]);
           break;
 
         case 'content-transfer-encoding':
           encoding = HeaderValue.parse(multipart.headers[key]);
           break;
 
         case 'content-disposition':
-          disposition = HeaderValue.parse(multipart.headers[key]);
+          disposition = HeaderValue.parse(multipart.headers[key],
+                                          preserveBackslash: true);
           break;
 
         default:
           remaining[key] = multipart.headers[key];
           break;
       }
     }
     if (disposition == null) {
       throw new HttpException(
           "Mime Multipart doesn't contain a Content-Disposition header value");
@@ skipping 46 matching lines @@
     while ((amp = input.indexOf('&', offset)) >= 0) {
       buffer.write(input.substring(offset, amp));
       int end = input.indexOf(';', amp);
       parse(amp, end);
       offset = end + 1;
     }
     buffer.write(input.substring(offset));
     return buffer.toString();
   }
 }