Use process.mixin instead of sys.mixin

The process namespace has not been cleaned up yet, so mixin is still
attached to process.

lib/fs.js

@@ -1,5 +1,3 @@
-var sys = require('sys');
-
 exports.Stats = process.Stats;
 
 process.Stats.prototype._checkModeProperty = function (property) {
@@ -391,7 +389,7 @@ var FileWriteStream = exports.FileWriteStream = function(path, options) {
   this.encoding = 'binary';
   this.mode = 0666;
 
-  sys.mixin(this, options || {});
+  process.mixin(this, options || {});
 
   var
     self = this,