pkg/http_server: move to multi-lib layout

pkg/http_server/lib/src/virtual_directory.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_directory;
 
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:mime/mime.dart';
+import 'package:path/path.dart';
 
 // Used for signal a directory redirecting, where a tailing slash is missing.
 class _DirectoryRedirect {
   const _DirectoryRedirect();
 }
 
 typedef dynamic _DirCallback(Directory dir, HttpRequest request);
 typedef dynamic _ErrorCallback(HttpRequest request);
 
 /**
@@ skipping 446 matching lines @@
 
   Future close() => new Future.value();
 
   void setMimeType(List<int> bytes) {
     var mimeType = lookupMimeType(path, headerBytes: bytes);
     if (mimeType != null) {
       response.headers.contentType = ContentType.parse(mimeType);
     }
   }
 }