events: check, if _events is an own property

[vkurchatkin]

Rework of #7158. Differences:

• instead of extending existing _events, just check if they are an own property;
• no performance impact in general case.

In case _events exists instead of !this.hasOwnProperty('_events') this._events === this.__proto__._events is used, which is slightly faster.

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[bnoordhuis]

Calling @trevnorris.

[vkurchatkin]

ping @trevnorris

[TooTallNate]

Is Object.getPrototypeOf() a lot slower than this.__proto__? We try to avoid using __proto__ in node core, and I'm pretty sure that currently we're achieving that (sans this patch).

[vkurchatkin]

Well, it appears like it's actually even faster. Updated.

btw __proto__ is still used here: https://github.com/joyent/node/blob/master/src/node.js#L33

[trevnorris]

Either way the patch makes the code path slower. Between 20-40ns on my i7. But since this is for a special case I'm not concerned about it. LGTM.

[trevnorris]

Merged in 2c6b424.