Revert "Remove Expect from core library."

tests/standalone/io/delete_symlink_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';
 
 void main() {
   // temp/
   //   a/
   //     file.txt
   //   b/
   //     a_link -> a
   var d = new Directory("").createTempSync();
@@ skipping 26 matching lines @@
     // We should not have recursed through a_link into a.
     Expect.isTrue(f.existsSync());
 
     // Clean up after ourselves.
     d.deleteSync(recursive: true);
 
     // Terminate now that we are done with everything.
     keepAlive.close();
   });
 }