Revert "Remove Expect from core library."

tests/standalone/io/socket_port_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:io";
 
 void testPort() {
   ServerSocket.bind().then((server) {
     Socket.connect("127.0.0.1", server.port).then((clientSocket) {
       server.listen((Socket socket) {
           Expect.equals(socket.port, server.port);
         Expect.equals(clientSocket.port, socket.remotePort);
         Expect.equals(clientSocket.remotePort, socket.port);
         server.close();
       });
     });
   });
 }
 
 void main() {
   testPort();
 }