Use toNativePath() in http_server pkg, to get the native path string on Windows.

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;
 
 /**
  * A [VirtualDirectory] can serve files and directory-listing from a root path,
  * to [HttpRequest]s.
  *
@@ skipping 69 matching lines @@
           if (entity is File) {
             entity.openRead().pipe(request.response).catchError((_) {});
           } else {
             _serveErrorPage(HttpStatus.NOT_FOUND, request);
           }
         });
   }
 
   Future<FileSystemEntity> _locateResource(Path path,
                                            Iterable<String> segments) {
-    return FileSystemEntity.type(path.toString(), followLinks: false)
+    return FileSystemEntity.type(path.toNativePath(), followLinks: false)
         .then((type) {
           switch (type) {
             case FileSystemEntityType.FILE:
               if (segments.isEmpty) return new File.fromPath(path);
               break;
 
             case FileSystemEntityType.DIRECTORY:
               if (segments.isEmpty) {
                 if (_allowDirectoryListing) return new Directory.fromPath(path);
               } else {
                 return _locateResource(path.append(segments.first),
                                        segments.skip(1));
               }
               break;
 
             case FileSystemEntityType.LINK:
               if (followLinks) {
                 // TODO
               }
               break;
-
           }
           // Return `null` on fall-through, to indicate NOT_FOUND.
           return null;
         });
   }
 
   void _serveErrorPage(int error, HttpRequest request) {
     request.response.statusCode = error;
     request.response.close();
   }
 }