fs: use arrow function for lexical this

Refs nodejs#7414

lib/fs.js

@@ -1708,20 +1708,19 @@ function ReadStream(path, options) {
 fs.FileReadStream = fs.ReadStream; // support the legacy name
 
 ReadStream.prototype.open = function() {
-  var self = this;
-  fs.open(this.path, this.flags, this.mode, function(er, fd) {
+  fs.open(this.path, this.flags, this.mode, (er, fd) => {
     if (er) {
-      if (self.autoClose) {
-        self.destroy();
+      if (this.autoClose) {
+        this.destroy();
       }
-      self.emit('error', er);
+      this.emit('error', er);
       return;
     }
 
-    self.fd = fd;
-    self.emit('open', fd);
+    this.fd = fd;
+    this.emit('open', fd);
     // start the flow of data.
-    self.read();
+    this.read();
   });
 };