Handle multipart MIME parsing with no parts

pkg/mime/test/mime_multipart_transformer_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.
 
 import 'dart:async';
 import 'dart:math';
 
 import "package:unittest/unittest.dart";
 import "package:mime/mime.dart";
 
@@ skipping 51 matching lines @@
     expect(Future.wait([
         testWrite(data),
         testWrite(data, 10),
         testWrite(data, 2),
         testWrite(data, 1)]),
         completes);
   });
 }
 
 void _testParseValid() {
+  // Empty message from Chrome form post.
+  var message = '------WebKitFormBoundaryU3FBruSkJKG0Yor1--\r\n';
+  _testParse(message, "----WebKitFormBoundaryU3FBruSkJKG0Yor1", [], []);
+
   // Sample from Wikipedia.
-  var message = """
+  message = """
 This is a message with multiple parts in MIME format.\r
 --frontier\r
 Content-Type: text/plain\r
 \r
 This is the body of the message.\r
 --frontier\r
 Content-Type: application/octet-stream\r
 Content-Transfer-Encoding: base64\r
 \r
 PGh0bWw+CiAgPGhlYWQ+CiAgPC9oZWFkPgogIDxib2R5PgogICAgPHA+VGhpcyBpcyB0aGUg
@@ skipping 217 matching lines @@
 \r
 Body2\r
 --xxx\r\n""";
   _testParse(message, "xxx", null, [null, null], true);
 }
 
 void main() {
   _testParseValid();
   _testParseInvalid();
 }