Easier access to individual file permission bits

[DartBot]

This issue was originally filed by @seaneagan

---

Currently FileStat has mode which is an opaque integer which according to the docs encodes the file permissions in its lower 16 bits, but it's not documented which bits correspond with which permissions. FileStat also has modeString which might be useful for display purposes, but not for actually checking individual file permissions bits.

There should be an API similar to:

enum FilePermissionRole { WORLD, GROUP, USER }
enum FilePermission { READ, WRITE, EXECUTE, STICKY, SET_UID, SET_GID }

bool hasPermission(FilePermissionRole role, FilePermission permission);

If FileStat.mode were better documented, then this could potentially be implemented in a package, instead of dart:io itself.

[floitschG]

Added Area-Library, Library-IO, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

---

I figured out the bit permission mapping from the implementation of modeString:

https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/sdk/lib/io/file_system_entity.dart#­158

So here's a draft implementation of the above API:

https://gist.github.com/seaneagan/e01ae92825694a4f4dc4