Remove EventEmitter#domain clobbering by 'domain' module.

[ashtuchkin]

Field clobbering is removed on 2 levels:

• The field itself is renamed to '_domain', according to conventions for internal field naming.
• The 'events.usingDomains' is checked so that programs that don't use 'domains' module are not affected even if they use '_domain' field.

Fixes #3922.
Test included.

[bnoordhuis]

/cc @isaacs

[isaacs]

So, you've just moved the problem from the domain field name to the _domain field name.

Either way, you have the exact same hazard. Except, now, rather than a public documented part of the EE api, it's something private. ("Why are you clobbering the private members of my class!?")

It would make more sense to me to check whether this.domain instanceof Domain perhaps, and avoid setting it if there's already something there. But just moving it from domain to _domain does not improve things.

[ashtuchkin]

I agree with that argumentation. The solution I proposed is not clean. One question - are you planning to make domain field official? It's not documented right now as far as I know.

If yes, then I think its a bad decision. It'll create lots of wtfs with existing code without providing lots of usefulness. Arguably, this is a domain module implementation detail and users will rarely need to access or modify this field.

If no, then I'd still suggest making it private, like _events. Maybe even use double underscore to show framework-level private-ness. This is in addition to the instanceof check you suggested.

[bentaber]

Pull #3998 solves this issue in a different way, using an instanceof check and leaving the .domain interface in tact, baselined off of the v0.8 branch. Just trying to bump the visibility on that one. Thanks.

/cc @isaacs