Fix test.dart multitest generated_test directory on Windows.

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 139 matching lines @@
            isNegative,
            isNegativeIfChecked,
            enableFatalTypeErrors);
   }
   multitestDone();
 }
 
 
 String CreateMultitestDirectory(String outputDir, String testDir) {
   final String generatedTestDirectory = 'generated_tests';
-  Directory parentDir = new Directory(outputDir + generatedTestDirectory);
+  Directory generatedTestDir = new Directory('$outputDir/generated_tests');
   if (!new Directory(outputDir).existsSync()) {
-    print('test.dart: build dir does not exist: $outputDir');
     new Directory(outputDir).createSync();
   }
-  if (!parentDir.existsSync()) {
-    print('test.dart: generated tests dir does not exist: ${parentDir.path}');
-    parentDir.createSync();
+  if (!generatedTestDir.existsSync()) {
+    generatedTestDir.createSync();
   }
   var split = testDir.split('/');
   var lastComponent = split.removeLast();
   Expect.isTrue(lastComponent == 'src');
-  String path = '${parentDir.path}/${split.last()}';
+  String path = '${generatedTestDir.path}/${split.last()}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return path;
 }