pkg/http_server: move to multi-lib layout

pkg/http_server/lib/src/http_multipart_form_data.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;
+library http_server.http_multipart_form_data;
 
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:mime/mime.dart';
+
+import 'http_multipart_form_data_impl.dart';
 
 /**
  * [:HttpMultipartFormData:] class used for 'upgrading' a [MimeMultipart] by
  * parsing it as a 'multipart/form-data' part. The following code shows how
  * it can be used.
  *
  *   HttpServer server = ...;
  *   server.listen((request) {
  *     String boundary = request.headers.contentType.parameters['boundary'];
  *     request
@@ skipping 52 matching lines @@
    * thrown.
    *
    * If the [MimeMultipart] is identified as text, and the [:Content-Type:]
    * header is missing, the data is decoded using [defaultEncoding]. See more
    * information in the
    * [HTML5 spec](http://dev.w3.org/html5/spec-preview/
    * constraints.html#multipart-form-data).
    */
   static HttpMultipartFormData parse(MimeMultipart multipart,
                                      {Encoding defaultEncoding: UTF8})
-      => _HttpMultipartFormData.parse(multipart, defaultEncoding);
+      => HttpMultipartFormDataImpl.parse(multipart, defaultEncoding);
 }