Really remove io support when dart:io is unsupported.

runtime/bin/stdio.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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 {
 
 void FUNCTION_NAME(Stdin_ReadByte)(Dart_NativeArguments args) {
   ScopedBlockingCall blocker;
   Dart_SetReturnValue(args, Dart_NewInteger(Stdin::ReadByte()));
 }
 
 
 void FUNCTION_NAME(Stdin_GetEchoMode)(Dart_NativeArguments args) {
@@ skipping 36 matching lines @@
     Dart_ListSetAt(list, 0, Dart_NewInteger(size[0]));
     Dart_ListSetAt(list, 1, Dart_NewInteger(size[1]));
     Dart_SetReturnValue(args, list);
   } else {
     Dart_SetReturnValue(args, DartUtils::NewDartOSError());
   }
 }
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)