Revert "Remove Expect from core library."

tests/standalone/io/file_system_links_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.
 
-import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
 
 
 createLink(String dst, String link, bool symbolic, void callback()) {
   var args = [ symbolic ? '-s' : '', dst, link ];
   var script = 'tests/standalone/io/ln.sh';
   if (!new File(script).existsSync()) {
     script = '../$script';
   }
@@ skipping 226 matching lines @@
 
 main() {
   testFileExistsCreate();
   testFileDelete();
   testFileWriteRead();
   testDirectoryExistsCreate();
   testDirectoryDelete();
   testDirectoryListing();
   testDirectoryListingBrokenLink();
 }