Fix windows bot failure in physical_file_system_test.dart.

pkg/front_end/test/physical_file_system_test.dart

 // Copyright (c) 2016, 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.
 // SharedOptions=--supermixin
 
 library front_end.test.physical_file_system_test;
 
+import 'dart:async';
 import 'dart:convert';
 import 'dart:io' as io;
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/physical_file_system.dart';
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
@@ skipping 181 matching lines @@
 
 class _BaseTest {
   io.Directory tempDirectory;
   String tempPath;
 
   setUp() {
     tempDirectory = io.Directory.systemTemp.createTempSync('test_file_system');
     tempPath = tempDirectory.absolute.path;
   }
 
-  tearDown() {
-    tempDirectory.deleteSync(recursive: true);
+  tearDown() async {
+    try {
+      tempDirectory.deleteSync(recursive: true);
+    } on io.FileSystemException {
+      // Sometimes on Windows the delete fails with errno 32
+      // (ERROR_SHARING_VIOLATION: The process cannot access the file because it
+      // is being used by another process).  Wait 1 second and try again.
+      await new Future.delayed(new Duration(seconds: 1));
+      tempDirectory.deleteSync(recursive: true);
+    }
   }
 }