Fix lowercasing output filenames for dart:3320 as well as fix for co19 test

lib/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');
 #import('dart:uri');
 
 // TODO(ahe): This library should be replaced by a general
 // path-munging library.
 //
 // See also:
 // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
 
 String nativeToUriPath(String filename) {
-  if (Platform.operatingSystem != 'windows') return filename;
-  filename = filename.toLowerCase();
-  filename = filename.replaceAll('\\', '/');
-  if (filename.length > 2 && filename[1] == ':') {
-    filename = "/$filename";
-  }
-  return filename;
+  return new Path.fromNative(filename).toString();
 }
 
 String uriPathToNative(String path) {
-  if (Platform.operatingSystem != 'windows') return path;
-  path = path.toLowerCase();
-  if (path.length > 3 && path[0] == '/' && path[2] == ':') {
-    return path.substring(1).replaceAll('/', '\\');
-  } else {
-    return path.replaceAll('/', '\\');
-  }
+  return new Path(path).toNativePath();
 }
 
 Uri getCurrentDirectory() {
   final String dir = nativeToUriPath(new File('.').fullPathSync());
   return new Uri.fromComponents(scheme: 'file', path: appendSlash(dir));
 }
 
 String appendSlash(String path) => path.endsWith('/') ? path : '$path/';