Fix bad rewrite of boolean expression

dart/sdk/lib/_internal/compiler/implementation/filenames.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.
 
 library filenames;
 
 import 'dart:io';
 
 // For information about how to convert Windows file names to URIs:
 // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
 
 String nativeToUriPath(String filename) {
   // TODO(ahe): It would be nice to use a Dart library instead.
-  if (Platform.isWindows) return filename;
+  if (!Platform.isWindows) return filename;
   filename = filename.replaceAll('\\', '/');
   if (filename.length > 2 && filename[1] == ':') {
     filename = "/$filename";
   }
   return filename;
 }
 
 String uriPathToNative(String path) {
   // TODO(ahe): It would be nice to use a Dart library instead.
-  if (Platform.operatingSystem != 'windows') return path;
+  if (!Platform.isWindows) return filename;
   if (path.length > 3 && path[0] == '/' && path[2] == ':') {
     return path.substring(1).replaceAll('/', '\\');
   } else {
     return path.replaceAll('/', '\\');
   }
 }
 
 final Uri currentDirectory = new Uri(
     scheme: 'file',
     path: appendSlash(nativeToUriPath(new File('.').fullPathSync())));
 
 String appendSlash(String path) => path.endsWith('/') ? path : '$path/';