Revert "Remove Expect from core library."

tests/lib/async/future_delayed_error_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.
 
 library future_delayed_error_test;
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:isolate';
 
 testImmediateError() {
   // An open ReceivePort keeps the VM running. If the error-handler below is not
   // executed then the test will fail with a timeout.
   var port = new ReceivePort();
   var future = new Future.immediateError("error");
   future.catchError((e) {
     port.close();
@@ skipping 13 matching lines @@
     port.close();
     Expect.equals(e.error, "foobar");
   });
 }
 
 main() {
   testImmediateError();
   testDelayedError();
 }