Really remove io support when dart:io is unsupported.

runtime/bin/socket_win.cc

Index: runtime/bin/socket_win.cc
diff --git a/runtime/bin/socket_win.cc b/runtime/bin/socket_win.cc
index ea306b7282740c38adc21c88155b958879bd8860..4534c8cedbbc3367a70398c36c8bcd4d73005ca9 100644
--- a/runtime/bin/socket_win.cc
+++ b/runtime/bin/socket_win.cc
@@ -2,6 +2,8 @@
 // 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.
 
+#if !defined(DART_IO_DISABLED)
+
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
@@ -712,3 +714,5 @@ bool Socket::LeaveMulticast(
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)
+
+#endif  // !defined(DART_IO_DISABLED)