Really remove io support when dart:io is unsupported.

runtime/bin/stdio.h

 // 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.
 
 #ifndef BIN_STDIO_H_
 #define BIN_STDIO_H_
 
+#if defined(DART_IO_DISABLED)
+#error "stdio.h can only be included on builds with IO enabled"
+#endif
+
 #include "bin/builtin.h"
 #include "bin/utils.h"
 
 #include "platform/globals.h"
 
 namespace dart {
 namespace bin {
 
 class Stdin {
  public:
@@ skipping 17 matching lines @@
 
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Stdout);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_STDIO_H_