Move MimeMultipartTransformer and HttpBodyHandler to mime and http_server packages.

pkg/http_server/lib/src/http_body.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;
+part of http_server;
 
 
 /**
  * [HttpBodyHandler] is a helper class for processing and collecting
  * HTTP message data in an easy-to-use [HttpBody] object. The content
  * body is parsed, depending on the `Content-Type` header field. When
  * the full body is read and parsed the body content is made
  * available. The class can be used to process both server requests
  * and client responses.
  *
@@ skipping 213 matching lines @@
    * The [ContentType] of the uploaded file. For 'text/\*' and
    * 'application/json' the [data] field will a String.
    */
   ContentType contentType;
 
   /**
    * The content of the file. Either a [String] or a [List<int>].
    */
   dynamic content;
 }