Revert "Remove Expect from core library."

tests/html/event_customevent_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 EventCustomEventTest;
-import "package:expect/expect.dart";
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 // TODO(nweiz): Make this private to testEvents when Frog supports closures with
 // optional arguments.
 eventTest(String name, Event eventFn(), void validate(Event),
     [String type = 'foo']) {
   test(name, () {
     final el = new Element.tag('div');
@@ skipping 19 matching lines @@
       fired = true;
       expect(ev.detail, 'detail');
     });
 
     var ev = new CustomEvent('foo', canBubble: false, cancelable: false,
         detail: 'detail');
     el.dispatchEvent(ev);
     expect(fired, isTrue);
   });
 }