Remove Expect from core library.

tests/standalone/io/socket_exception_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.
 //
 // Tests socket exceptions.
 
+import "package:expect/expect.dart";
 import "dart:async";
 import "dart:isolate";
 import "dart:io";
 
 class SocketExceptionTest {
 
   static void serverSocketExceptionTest() {
     bool exceptionCaught = false;
     bool wrongExceptionCaught = false;
 
@@ skipping 215 matching lines @@
     clientSocketAddCloseErrorTest();
     clientSocketAddCloseResultErrorTest();
     unknownHostTest();
   }
 }
 
 main() {
   SocketExceptionTest.testMain();
 }