Remove Expect from core library.

tests/standalone/io/http_headers_state_test.dart

 // Copyright (c) 2012, 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 server = new HttpServer();
   server.onError = (e) => Expect.fail("Unexpected error $e");
   server.listen("127.0.0.1", 0, backlog: totalConnections);
   server.defaultRequestHandler = (HttpRequest request, HttpResponse 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");
 }