Really remove io support when dart:io is unsupported.

runtime/bin/directory.cc

 // Copyright (c) 2012, 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/directory.h"
 
 #include "bin/dartutils.h"
 #include "include/dart_api.h"
 #include "platform/assert.h"
 
 namespace dart {
 namespace bin {
 
 void FUNCTION_NAME(Directory_Current)(Dart_NativeArguments args) {
@@ skipping 400 matching lines @@
   if (listing->error()) {
     listing->HandleError();
     listing->HandleDone();
   } else {
     while (ListNext(listing)) {}
   }
 }
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)