Revert "Remove Expect from core library."

tests/json/json_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 json_tests;
-import "package:expect/expect.dart";
 import 'dart:json' as json;
 import 'dart:html';
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 
 main() {
   useHtmlConfiguration();
   test('Parse', () {
     // Scalars.
     expect(json.parse(' 5 '), equals(5));
@@ skipping 136 matching lines @@
 
 /**
  * Checks that the argument can be converted to a JSON string and
  * back, and produce something equivalent to the argument.
  */
 validateRoundTrip(expected) {
   expect(json.parse(json.stringify(expected)), equals(expected));
 }