Revert "Remove Expect from core library."

tests/html/documentfragment_test.dart

 // Copyright (c) 2011, 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 DocumentFragmentTest;
-import "package:expect/expect.dart";
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'util.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   var isAnchorElement =
       predicate((x) => x is AnchorElement, 'is an AnchorElement');
@@ skipping 155 matching lines @@
     expect(fragment.innerHtml, "foo<a>bar</a>");
   });
 
   test('query searches the fragment', () {
     var fragment = new DocumentFragment.html(
       "<div class='foo'><a>foo</a><b>bar</b></div>");
     expect(fragment.query(".foo a").tagName, "A");
     expect(_nodeStrings(fragment.queryAll(".foo *")), equals(["A", "B"]));
   });
 }