Really remove io support when dart:io is unsupported.

runtime/bin/socket_linux.cc

Index: runtime/bin/socket_linux.cc
diff --git a/runtime/bin/socket_linux.cc b/runtime/bin/socket_linux.cc
index b9068d770d57729cb6868a8c26535b3eeda4d810..1befb691c6c78f25d124496bfeb1eb83d49900e4 100644
--- a/runtime/bin/socket_linux.cc
+++ b/runtime/bin/socket_linux.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_LINUX)
 
@@ -585,3 +587,5 @@ bool Socket::LeaveMulticast(
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)
+
+#endif  // !defined(DART_IO_DISABLED)