fs: define constants with const

Define fs constants using const, as the newer version of
v8 supports it, and appears to be capable of optimizing.

PR-URL: #522
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/fs.js

@@ -16,21 +16,21 @@ var FSReqWrap = binding.FSReqWrap;
 var Readable = Stream.Readable;
 var Writable = Stream.Writable;
 
-var kMinPoolSpace = 128;
-var kMaxLength = require('smalloc').kMaxLength;
-
-var O_APPEND = constants.O_APPEND || 0;
-var O_CREAT = constants.O_CREAT || 0;
-var O_EXCL = constants.O_EXCL || 0;
-var O_RDONLY = constants.O_RDONLY || 0;
-var O_RDWR = constants.O_RDWR || 0;
-var O_SYNC = constants.O_SYNC || 0;
-var O_TRUNC = constants.O_TRUNC || 0;
-var O_WRONLY = constants.O_WRONLY || 0;
-var F_OK = constants.F_OK || 0;
-var R_OK = constants.R_OK || 0;
-var W_OK = constants.W_OK || 0;
-var X_OK = constants.X_OK || 0;
+const kMinPoolSpace = 128;
+const kMaxLength = require('smalloc').kMaxLength;
+
+const O_APPEND = constants.O_APPEND || 0;
+const O_CREAT = constants.O_CREAT || 0;
+const O_EXCL = constants.O_EXCL || 0;
+const O_RDONLY = constants.O_RDONLY || 0;
+const O_RDWR = constants.O_RDWR || 0;
+const O_SYNC = constants.O_SYNC || 0;
+const O_TRUNC = constants.O_TRUNC || 0;
+const O_WRONLY = constants.O_WRONLY || 0;
+const F_OK = constants.F_OK || 0;
+const R_OK = constants.R_OK || 0;
+const W_OK = constants.W_OK || 0;
+const X_OK = constants.X_OK || 0;
 
 var isWindows = process.platform === 'win32';