EventEmitter: move constructor guts to EventEmitter.init

[piscisaureus]

After landing 6ed861d it is no longer possible to reliably monkey-patch
the EventEmitter constructor. However there's valid use cases for that,
and makes for easier debugging. Therefore, move the guts of the
constructor to a separate function which is monkey-patchable.

The idea was signed off by @isaacs already. Here's the implementation.
suggested reviewers: @trevnorris @tjfontaine

[Nodejs-Jenkins]

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[piscisaureus]

^-- Okay, jankins, there you go.

[trevnorris]

I now realize that both this and our current implementation is broken. If a user runs EventEmitter(); then it's going to add all the object properties to global. That's incorrect.

[trevnorris]

This isn't necessary. this._maxListeners is already set to undefined by the prototype.

[piscisaureus]

I copied this code literally from the original constructor. I think it's better for performance to have the property on the object itself.

[piscisaureus]

Also see cf0fa96

[piscisaureus]

If you care, however, I'll drop that line in one go :). Just say the word.

[trevnorris]

IMO, we shouldn't consider "monkey-patching" a useful or good practice.

This is still a little ways off, but hence why I'm finishing up #6502. It's going to be necessary anyways to fix the edge cases from using AsyncListeners, and it provides a performance gain not having to check the .domain object property all the time.

[piscisaureus]

@trevnorris Not a good or useful practice per se. But it's very useful for debugging or for making new stuff work in older versions. Think about the asyncListener polyfill that Tim has done, for example.

[trevnorris]

LGTM