debugger: use arrow function for lexical this

Refs: #7414
PR-URL: #7415
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

lib/_debug_agent.js

@@ -50,9 +50,8 @@ function Agent() {
   this.binding = process._debugAPI;
   assert(this.binding, 'Debugger agent running without bindings!');
 
-  var self = this;
-  this.binding.onmessage = function(msg) {
-    self.clients.forEach(function(client) {
+  this.binding.onmessage = (msg) => {
+    this.clients.forEach((client) => {
       client.send({}, msg);
     });
   };
@@ -67,11 +66,10 @@ Agent.prototype.onConnection = function onConnection(socket) {
   c.start();
   this.clients.push(c);
 
-  var self = this;
-  c.once('close', function() {
-    var index = self.clients.indexOf(c);
+  c.once('close', () => {
+    var index = this.clients.indexOf(c);
     assert(index !== -1);
-    self.clients.splice(index, 1);
+    this.clients.splice(index, 1);
   });
 };
 
@@ -98,9 +96,8 @@ function Client(agent, socket) {
 
   this.on('data', this.onCommand);
 
-  var self = this;
-  this.socket.on('close', function() {
-    self.destroy();
+  this.socket.on('close', () => {
+    this.destroy();
   });
 }
 util.inherits(Client, Transform);