Revert "Remove Expect from core library."

tests/standalone/io/directory_invalid_arguments_test.dart

 // Copyright (c) 2012, 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 testFailingList(Directory d, var recursive) {
   var port = new ReceivePort();
   int errors = 0;
   d.list(recursive: recursive).listen(
     () => Expect.fail("Unexpected listing result"),
     onError: (error) {
       errors += 1;
@@ skipping 30 matching lines @@
                 (e) => e is ArgumentError);
   d = new Directory(".");
   testFailingList(d, 1);
   Expect.throws(() => d.listSync(recursive: 1),
                 (e) => e is ArgumentError);
 }
 
 main() {
   testInvalidArguments();
 }