Really remove io support when dart:io is unsupported.

runtime/bin/io_service_unsupported.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/builtin.h"
 #include "bin/dartutils.h"
-
 #include "include/dart_api.h"
 
 namespace dart {
 namespace bin {
 
 void FUNCTION_NAME(IOService_NewServicePort)(Dart_NativeArguments args) {
   Dart_ThrowException(DartUtils::NewDartArgumentError(
       "IOService is unsupported on this platform"));
 }
 
 }  // namespace bin
 }  // namespace dart
 
+#endif  // defined(DART_IO_DISABLED)