Really remove io support when dart:io is unsupported.

runtime/bin/stdio.cc

Index: runtime/bin/stdio.cc
diff --git a/runtime/bin/stdio.cc b/runtime/bin/stdio.cc
index b14fe9212b4de09ebd49507640d2c1ba5a30cad2..47bd55478ee8f7fbf913abfc7e4daaa20d50be29 100644
--- a/runtime/bin/stdio.cc
+++ b/runtime/bin/stdio.cc
@@ -2,17 +2,19 @@
 // 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 "bin/stdio.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/utils.h"
 
+#include "include/dart_api.h"
+
 #include "platform/globals.h"
 #include "platform/utils.h"
 
-#include "include/dart_api.h"
-
 namespace dart {
 namespace bin {
 
@@ -69,3 +71,5 @@ void FUNCTION_NAME(Stdout_GetTerminalSize)(Dart_NativeArguments args) {
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)