Test scripts: Replace a guard that was optimistically removed, fixing a break on Windows.

tools/testing/dart/multitest.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("multitest");
 
 #import("dart:io");
 #import("test_suite.dart");
 
 // Multitests are Dart test scripts containing lines of the form
@@ skipping 197 matching lines @@
   Path sourceDir = filePath.directoryPath;
   Path targetDir = CreateMultitestDirectory(outputDir, suiteDir);
   Expect.isNotNull(targetDir);
 
   // Copy all the relative imports of the multitest.
   Set<Path> importsToCopy = _findAllRelativeImports(filePath);
   List<Future> futureCopies = [];
   for (Path importPath in importsToCopy) {
     // Make sure the target directory exists.
     Path importDir = importPath.directoryPath;
-    TestUtils.mkdirRecursive(targetDir, importDir);
+    if (!importDir.isEmpty) {
+      TestUtils.mkdirRecursive(targetDir, importDir);
+    }
     // Copy file.
     futureCopies.add(TestUtils.copyFile(sourceDir.join(importPath),
                                         targetDir.join(importPath)));
   }
 
   // Wait until all imports are copied before scheduling test cases.
   Futures.wait(futureCopies).then((ignored) {
     String baseFilename = filePath.filenameWithoutExtension;
     for (String key in tests.getKeys()) {
       final Path multitestFilename =
@@ skipping 38 matching lines @@
     // TestSuite.forDirectory.
     split.removeLast();
   }
   String path = '${generatedTestDir.path}/${split.last()}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return new Path.fromNative(new File(path).fullPathSync());
 }