Revert "Remove Expect from core library."

tests/standalone/io/https_client_exception_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";
 import "dart:isolate";
 import "dart:uri";
 
 void testBadHostName() {
   HttpClient client = new HttpClient();
   ReceivePort port = new ReceivePort();
   client.getUrl(Uri.parse("https://some.bad.host.name.7654321/"))
       .then((HttpClientRequest request) {
         Expect.fail("Should not open a request on bad hostname");
       })
       .catchError((error) {
         port.close();  // Should throw an error on bad hostname.
       });
 }
 
 void InitializeSSL() {
   SecureSocket.initialize();
 }
 
 void main() {
   testBadHostName();
   Expect.throws(InitializeSSL);
 }