Really remove io support when dart:io is unsupported.

runtime/bin/directory.cc

Index: runtime/bin/directory.cc
diff --git a/runtime/bin/directory.cc b/runtime/bin/directory.cc
index c2f3dc82c968023a96ebeec042a2689903ae691a..982a4be0147d5ffad9e0696042a6f8b741c990e5 100644
--- a/runtime/bin/directory.cc
+++ b/runtime/bin/directory.cc
@@ -2,6 +2,8 @@
 // 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"
@@ -422,3 +424,5 @@ void Directory::List(DirectoryListing* listing) {
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)