Summary: The `_activeListenerCount` variable is overkill, we should be able to achieve the same result using `Object.keys(this._listeners).length`.
Test Plan:
Mucked around in a NodeJS shell.
```lang=js
> Object.keys({}).length
0
> Object.keys({foo: 'bar'}).length
1
> Object.keys({1: 'foo', 2: 'bar'}).length
2
```
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9554
279a0e5371