cluster: dont rely on `this` in `fork`

PR-URL: https://github.com/nodejs/node/pull/5216
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

lib/cluster.js

@@ -342,7 +342,7 @@ function masterInit() {
     });
 
     worker.on('message', (message, handle) =>
-      this.emit('message', message, handle)
+      cluster.emit('message', message, handle)
     );
 
     worker.process.once('exit', function(exitCode, signalCode) {

test/parallel/test-cluster-disconnect-idle-worker.js

@@ -2,10 +2,11 @@
 var common = require('../common');
 var assert = require('assert');
 var cluster = require('cluster');
+var fork = cluster.fork;
 
 if (cluster.isMaster) {
-  cluster.fork();
-  cluster.fork();
+  fork(); // it is intentionally called `fork` instead of
+  fork(); // `cluster.fork` to test that `this` is not used
   cluster.disconnect(common.mustCall(function() {
     assert.deepEqual(Object.keys(cluster.workers), []);
   }));