fs: use arrow function for lexical this

This commit merits extra scrutiny.

Refs nodejs#7414

lib/fs.js

@@ -1755,28 +1755,25 @@ ReadStream.prototype._read = function(n) {
     return this.push(null);
 
   // the actual read.
-  var self = this;
-  fs.read(this.fd, pool, pool.used, toRead, this.pos, onread);
-
-  // move the pool positions, and internal position for reading.
-  if (this.pos !== undefined)
-    this.pos += toRead;
-  pool.used += toRead;
-
-  function onread(er, bytesRead) {
+  fs.read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
     if (er) {
-      if (self.autoClose) {
-        self.destroy();
+      if (this.autoClose) {
+        this.destroy();
       }
-      self.emit('error', er);
+      this.emit('error', er);
     } else {
       var b = null;
       if (bytesRead > 0)
         b = thisPool.slice(start, start + bytesRead);
 
-      self.push(b);
+      this.push(b);
     }
-  }
+  });
+
+  // move the pool positions, and internal position for reading.
+  if (this.pos !== undefined)
+    this.pos += toRead;
+  pool.used += toRead;
 };