[dart/vfs] Allow POSIX compatibility flag.

This new flag is being passed but is not supported by Dart PseudoDirs.
This change supports the new flag with no additional checks to allow hub
listing code to work.

MS-2342: #done

Change-Id: Ic42d02e7aeda02d012c546f927f90c217195e8e7
diff --git a/public/dart/fuchsia_vfs/lib/src/pseudo_dir.dart b/public/dart/fuchsia_vfs/lib/src/pseudo_dir.dart
index 7279ab6..a29de56 100644
--- a/public/dart/fuchsia_vfs/lib/src/pseudo_dir.dart
+++ b/public/dart/fuchsia_vfs/lib/src/pseudo_dir.dart
@@ -203,7 +203,8 @@
         openRightWritable |
         openFlagDirectory |
         openFlagNodeReference |
-        openFlagDescribe;
+        openFlagDescribe |
+        openFlagPosix;
     var prohibitedFlags = openFlagCreate |
         openFlagCreateIfAbsent |
         openFlagTruncate |