Make test scripts work on Windows again.

tools/testing/dart/multitest.dart

 // Copyright (c) 2011, 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("multitest");
 
 // Multitests are Dart test scripts containing lines of the form
 // " [some dart code] /// [key]: [error type]"
 //
 // For each key in the file, a new test file is made containing all
@@ skipping 179 matching lines @@
                                    supportsFatalTypeErrors,
                                    currentKey,
                                    basePath,
                                    doTest,
                                    done);
   };
   file.create();
 }
 
 String CreateMultitestDirectory(String buildDir, String testDir) {
-  final String generatedTestDirectory = 'generated_tests/';
+  final String generatedTestDirectory = 'generated_tests';
   Directory parentDir = new Directory(buildDir + generatedTestDirectory);
   if (!parentDir.existsSync()) {
     parentDir.createSync();
   }
-  var split = testDir.split(new Platform().pathSeparator());
+  var split = testDir.split('/');
   var lastComponent = split.removeLast();
   Expect.isTrue(lastComponent == 'src');
-  String path = parentDir.path + split.last();
+  String path = '${parentDir.path}/${split.last()}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return path;
 }