Revert "Remove Expect from core library."

tests/standalone/io/http_connection_header_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 setConnectionHeaders(HttpHeaders headers) {
   headers.add(HttpHeaders.CONNECTION, "my-connection-header1");
   headers.add("My-Connection-Header1", "some-value1");
   headers.add(HttpHeaders.CONNECTION, "my-connection-header2");
   headers.add("My-Connection-Header2", "some-value2");
 }
 
@@ skipping 56 matching lines @@
         });
     }
   });
 }
 
 
 void main() {
   test(2, false);
   test(2, true);
 }