Really remove io support when dart:io is unsupported.

runtime/bin/socket_android.cc

Index: runtime/bin/socket_android.cc
diff --git a/runtime/bin/socket_android.cc b/runtime/bin/socket_android.cc
index 2d43a446e229dc9ed43f75486b43d82e7b5726c3..fd96a9af166d4cace5a37de991191148df12f48f 100644
--- a/runtime/bin/socket_android.cc
+++ b/runtime/bin/socket_android.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_ANDROID)
 
@@ -569,3 +571,5 @@ bool Socket::LeaveMulticast(
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_ANDROID)
+
+#endif  // !defined(DART_IO_DISABLED)