Make unittests work on Windows (again)

tests/compiler/dart2js/dart2js_batch2_test.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:async_helper/async_helper.dart';
@@ skipping 21 matching lines @@
       .then((Directory dir) {
     return dir;
   });
 }
 
 Future setup() {
   return createTempDir().then((Directory directory) {
     tmpDir = directory;
     String newPath = path.join(directory.path, "dart2js_batch2_run.dart");
     File source =
-        new File(Platform.script.resolve("dart2js_batch2_run.dart").path);
+        new File.fromUri(Platform.script.resolve("dart2js_batch2_run.dart"));
     source.copySync(newPath);
   });
 }
 
 void cleanUp() {
   print("Deleting '${tmpDir.path}'.");
   tmpDir.deleteSync(recursive: true);
 }
 
 Future launchDart2Js(_) {
@@ skipping 24 matching lines @@
 
 void main() {
   var tmpDir;
   asyncTest(() {
     return setup()
         .then(launchDart2Js)
         .then(runTests)
         .whenComplete(cleanUp);
   });
 }