Fix test.dart multitest generated_test directory on Windows.

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index 9599fa370e3f22e41fa7b65a35eabdf63ab348bc..02535797c7a6c89cdbcbb73cf65abb26d3b5da09 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -157,19 +157,17 @@ void DoMultitest(String filename,
 
 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();