Revert "Remove Expect from core library."

tests/standalone/io/http_content_length_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 testNoBody(int totalConnections, bool explicitContentLength) {
   var errors = 0;
   HttpServer.bind("127.0.0.1", 0, totalConnections).then((server) {
     server.listen(
         (HttpRequest request) {
           Expect.equals("0", request.headers.value('content-length'));
           Expect.equals(0, request.contentLength);
@@ skipping 217 matching lines @@
 
 void main() {
   testNoBody(5, false);
   testNoBody(5, true);
   testBody(5, false);
   testBody(5, true);
   testBodyChunked(5, false);
   testBodyChunked(5, true);
   testSetContentLength();
 }