Revert "Remove Expect from core library."

tests/html/svgelement_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 SvgElementTest;
-import "package:expect/expect.dart";
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import 'dart:svg' as svg;
 
 main() {
   useHtmlIndividualConfiguration();
 
   var isSvgSvgElement =
       predicate((x) => x is svg.SvgSvgElement, 'is a SvgSvgElement');
@@ skipping 399 matching lines @@
       var el = new svg.CircleElement();
       var classes = el.classes;
       expect(el.classes.length, 0);
       classes.toggle('foo');
       expect(el.classes.length, 1);
       classes.toggle('foo');
       expect(el.classes.length, 0);
     });
   });
 }