Revert "Remove Expect from core library."

tests/standalone/io/http_headers_state_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:isolate";
 import "dart:io";
 
 void test(int totalConnections, [String body]) {
   HttpServer.bind().then((server) {
 
     server.listen((HttpRequest request) {
       HttpResponse response = request.response;
       // Cannot mutate request headers.
       Expect.throws(() => request.headers.add("X-Request-Header", "value"),
@@ skipping 55 matching lines @@
         });
     }
 
   });
 }
 
 void main() {
   test(5);
   test(5, "Hello and goodbye");
 }