Really remove io support when dart:io is unsupported.

runtime/bin/root_certificates_unsupported.cc

Index: runtime/bin/root_certificates_unsupported.cc
diff --git a/runtime/bin/root_certificates_unsupported.cc b/runtime/bin/root_certificates_unsupported.cc
index b1a65f85cfa4d64f01e8e7706775e4862125bfbc..a899e3c2a5b39364f87431ae3b96b95290ae2737 100644
--- a/runtime/bin/root_certificates_unsupported.cc
+++ b/runtime/bin/root_certificates_unsupported.cc
@@ -2,7 +2,7 @@
 // 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_SECURE_SOCKET_DISABLED)
+#if !defined(DART_IO_DISABLED) && !defined(DART_IO_SECURE_SOCKET_DISABLED)
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_MACOS) || defined(TARGET_OS_ANDROID)
@@ -18,4 +18,5 @@ unsigned int root_certificates_pem_length = 0;
 
 #endif  // defined(TARGET_OS_MACOS) || defined(TARGET_OS_ANDROID)
 
-#endif  // !defined(DART_IO_SECURE_SOCKET_DISABLED)
+#endif  // !defined(DART_IO_DISABLED) &&
+        // !defined(DART_IO_SECURE_SOCKET_DISABLED)