events: make eventNames() use Reflect.ownKeys()

Use `Reflect.ownKeys()` instead of `Object.keys()` and
`Object.getOwnPropertySymbols()`.

PR-URL: nodejs#5822
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/events.js

@@ -437,12 +437,7 @@ function listenerCount(type) {
 }
 
 EventEmitter.prototype.eventNames = function eventNames() {
-  if (this._eventsCount > 0) {
-    const events = this._events;
-    return Object.keys(events).concat(
-      Object.getOwnPropertySymbols(events));
-  }
-  return [];
+  return this._eventsCount > 0 ? Reflect.ownKeys(this._events) : [];
 };
 
 // About 1.5x faster than the two-arg version of Array#splice().