Revert "Remove Expect from core library."

tests/standalone/io/directory_list_nonexistent_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.
 //
 // Directory listing test that tests listSync on a missing directory.
 //
 // TODO(7157): Merge this test into directory_test.dart testListNonExistent()
 // when it no longer crashes on Windows, when issue 7157 is resolved.
 
-import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
 
 void testListNonExistent() {
   var keepAlive = new ReceivePort();
   new Directory("").createTemp().then((d) {
     d.delete().then((ignore) {
       Expect.throws(() => d.listSync(), (e) => e is DirectoryIOException);
       Expect.throws(() => d.listSync(recursive: true),
                     (e) => e is DirectoryIOException);
@@ skipping 23 matching lines @@
       d.deleteSync(recursive: true);
       keepAlive.close();
     });
   });
 }
 
 void main() {
   testListNonExistent();
   testListTooLongName();
 }