Remove extra windows-char-sanitization, duplicationg and breaking the one in Uri.file.

runtime/bin/builtin.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 builtin;
 import 'dart:io';
 import 'dart:async';
 import 'dart:convert';
 // import 'root_library'; happens here from C Code
 
@@ skipping 195 matching lines @@
   }
   if (cwd.length > 1 && cwd[1] == ':') {
     return '/${cwd[0]}:';
   }
   return null;
 }
 
 
 void _setWorkingDirectory(cwd) {
   _workingWindowsDrivePrefix = _extractDriveLetterPrefix(cwd);
-  cwd = _sanitizeWindowsPath(cwd);
-  cwd = _enforceTrailingSlash(cwd);
   _workingDirectoryUri = new Uri.file(cwd);
   if (!_workingDirectoryUri.path.endsWith("/")) {
     var directoryPath = _workingDirectoryUri.path + "/";
     _workingDirectoryUri = _workingDirectoryUri.resolve(directoryPath);
   }
 
   _logResolution('# Working Directory: $cwd');
 }
 
 
@@ skipping 226 matching lines @@
   } else if (Platform.isWindows) {
     filename = '$name.dll';
   } else {
     _logResolution(
         'Native extensions not supported on ${Platform.operatingSystem}');
     throw 'Native extensions not supported on ${Platform.operatingSystem}';
   }
 
   return [path, filename, name];
 }