Revert "Remove Expect from core library."

tests/standalone/io/directory_create_race_test.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.
 //
 // Test for a race condition that can occur when recursively creating
 // a directory multiple times simultaneously.  This consistently reproduces
 // issue https://code.google.com/p/dart/issues/detail?id=7679 in revisions
 // without the fix for this issue.
 
-import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 void testCreateRecursiveRace() {
   var keepAlive = new ReceivePort();
   var temp = new Directory('').createTempSync();
   var d = new Directory('${temp.path}/a/b/c/d/e');
   Future.wait([
       d.create(recursive: true),
@@ skipping 14 matching lines @@
     temp.delete(recursive: true).then((_) {
       keepAlive.close();
     });
   });
 }
 
 void main() {
   testCreateRecursiveRace();
   testCreateRecursiveRace();
 }