Work around issue 9252 in scheduled_test.

pkg/scheduled_test/test/utils.dart

 // Copyright (c) 2013, 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 test_utils;
 
+import 'dart:io';
 import 'dart:async';
 
 import '../lib/src/utils.dart';
 import '../lib/src/mock_clock.dart' as mock_clock;
 
 export '../lib/src/utils.dart';
 
 /// Wraps [input] to provide a timeout. If [input] completes before
 /// [milliseconds] have passed, then the return value completes in the same way.
 /// However, if [milliseconds] pass before [input] has completed, [onTimeout] is
@@ skipping 20 matching lines @@
 }
 
 /// Returns a [Future] that will complete in [milliseconds].
 Future sleep(int milliseconds) {
   var completer = new Completer();
   mock_clock.newTimer(new Duration(milliseconds: milliseconds), () {
     completer.complete();
   });
   return completer.future;
 }
+
+// TODO(nweiz): Remove this when issue 9252 is fixed.
+/// Asynchronously recursively deletes [dir]. Returns a [Future] that completes
+/// when the deletion is done.
+Future deleteDir(String dir) =>
+  _attemptRetryable(() => new Directory(dir).delete(recursive: true));
+
+/// On Windows, we sometimes get failures where the directory is still in use
+/// when we try to do something with it. This is usually because the OS hasn't
+/// noticed yet that a process using that directory has closed. To be a bit
+/// more resilient, we wait and retry a few times.
+///
+/// Takes a [callback] which returns a future for the operation being attempted.
+/// If that future completes with an error, it will slepp and then [callback]
+/// will be invoked again to retry the operation. It will try a few times before
+/// giving up.
+Future _attemptRetryable(Future callback()) {
+  // Only do lame retry logic on Windows.
+  if (Platform.operatingSystem != 'windows') return callback();
+
+  var attempts = 0;
+  makeAttempt(_) {
+    attempts++;
+    return callback().catchError((e) {
+      if (attempts >= 10) {
+        throw 'Could not complete operation. Gave up after $attempts attempts.';
+      }
+
+      return sleep(500).then(makeAttempt);
+    });
+  }
+
+  return makeAttempt(null);
+}