Move MimeMultipartTransformer and HttpBodyHandler to mime and http_server packages.

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 dart.io;
+part of http_server;
 
 
 /**
  * [: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'];
@@ skipping 48 matching lines @@
   String value(String name);
 
   /**
    * Parse a [MimeMultipart] and return a [HttpMultipartFormData]. If the
    * [:Content-Disposition:] header is missing or invalid, a [HttpException] is
    * thrown.
    */
   static HttpMultipartFormData parse(MimeMultipart multipart)
       => _HttpMultipartFormData.parse(multipart);
 }