Revert "Remove Expect from core library."

tests/standalone/io/secure_socket_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
-import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 
 Future<HttpServer> startServer() {
   return HttpServer.bindSecure(
       "127.0.0.1",
       0,
       backlog: 5,
       certificateName: 'localhost_cert').then((server) {
     server.listen((HttpRequest request) {
@@ skipping 32 matching lines @@
         onDone: () {
           Expect.isTrue(body.length > 100, "$body\n${body.length}");
           Expect.equals(72, body[0]);
           Expect.equals(9, body[body.length - 1]);
           server.close();
         },
         onError: (e) => Expect.fail("Unexpected error $e"));
     });
   });
 }