Really remove io support when dart:io is unsupported.

runtime/bin/stdio_linux.cc

Index: runtime/bin/stdio_linux.cc
diff --git a/runtime/bin/stdio_linux.cc b/runtime/bin/stdio_linux.cc
index 8f51e7e7889934abae973f57a6af85e6488f3438..0634fa49821dfd8036d07f9dcdaaf74228176645 100644
--- a/runtime/bin/stdio_linux.cc
+++ b/runtime/bin/stdio_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)
 
@@ -79,3 +81,5 @@ bool Stdout::GetTerminalSize(intptr_t fd, int size[2]) {
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)
+
+#endif  // !defined(DART_IO_DISABLED)