Revert "Remove Expect from core library."

tests/standalone/io/http_session_test.dart

 // Copyright (c) 2013, 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.
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 
 const SESSION_ID = "DARTSESSID";
 
 String getSessionId(List<Cookie> cookies) {
   var id = cookies.reduce(null, (last, cookie) {
     if (last != null) return last;
     if (cookie.name.toUpperCase() == SESSION_ID) {
       Expect.isTrue(cookie.httpOnly);
@@ skipping 132 matching lines @@
         });
       });
   });
 }
 
 void main() {
   testSessions(1);
   testTimeout(5);
   testSessionsData();
 }