Remove Expect from core library.

tests/standalone/io/regress_7097_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:async';
 import 'dart:io';
 import 'dart:uri';
 
 void main() {
   var client = new HttpClient();
   var server = new HttpServer();
 
   var count = 0;
   server.defaultRequestHandler = (HttpRequest request, HttpResponse response) {
@@ skipping 58 matching lines @@
 
   makeRequest(0).then((_) {
     makeRequest(1).then((_) {
       makeRequest(2).then((_) {
         client.shutdown();
         server.close();
       });
     });
   });
 }