Mark standalone/io/socket_stream_close_test as flaky on windows.

tests/standalone/standalone.status

 # 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.
 
 package/invalid_uri_test: Fail, OK # Fails intentionally
 
 [ $runtime == vm ]
 package/package_isolate_test: Fail # Issue 7520.
 io/web_socket_test: Fail, Pass # Issue 8495.
 
@@ skipping 27 matching lines @@
 # from the VM before comparing it to the decomposed utf-8 used for the
 # file system.
 io/file_non_ascii_sync_test: Pass, Fail
 io/file_non_ascii_test: Pass, Fail
 io/directory_non_ascii_sync_test: Pass, Fail
 io/directory_non_ascii_test: Pass, Fail
 io/process_non_ascii_test: Pass, Fail
 
 [ $runtime == vm && $system == windows ]
 io/file_system_links_test: Skip  # No links on Windows.
+io/socket_stream_close_test: Fail, Pass # Issue 8556.
 
 [ $compiler == none && $runtime == drt ]
 io/*: Skip # Don't run tests using dart:io in the browser
 crypto/*: Skip # Don't run tests using dart:io in the browser
 package/*: Skip # Do not run those in Dartium.
 debugger/*: Skip # Do not run standalone debugger tests in browser.
 
 [ $compiler == dartc ]
 
 # package test issue 7392
@@ skipping 57 matching lines @@
 *: Skip
 
 [ $arch == simarm ]
 *: Skip
 
 [ $arch == mips ]
 *: Skip
 
 [ $arch == simmips ]
 *: Skip