pkg/http_server: move to multi-lib layout

pkg/http_server/lib/src/virtual_host.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 http_server;
+library http_server.virtual_host;
 
+import 'dart:async';
+import 'dart:io';
 
 /**
  * The [VirtualHost] class is a utility class for handling multiple hosts on
  * multiple sources, by using a named-based approach.
  */
 abstract class VirtualHost {
   /**
    * Get the [Stream] of [HttpRequest]s, not matching any hosts. If unused, the
    * default implementation will result in a [HttpHeaders.FORBIDDEN] response.
    */
@@ skipping 123 matching lines @@
 
   void _unhandled(HttpRequest request) {
     if (_unhandledController != null) {
       _unhandledController.add(request);
       return;
     }
     request.response.statusCode = HttpStatus.FORBIDDEN;
     request.response.close();
   }
 }