ES6 feature introduction: classes

[eakuefner]

Since @zeptonaut has initiated a procedure for opening up the tracing codebase to new ES6 features, we want to try classes next.

I'm doing some refactoring on Numeric right now, and it occurred to me that this would probably be a good place to start, since numerics are used by a fair amount of unit tested code, but not in so many places that any refactoring that has to be done would be insurmountable.

[zeptonaut]

Awesome! Thanks so much for doing this Ethan! I'd love to hear more as progress is made.

[zeptonaut]

Hey Ethan, any update on when you might be able to try this out?

[eakuefner]

It turns out that Numeric is not really a good place to start because it's heavily in flux right now.

I'm happy to push on this but we should find another place to do it -- I think you had another small class in mind?

[zeptonaut]

(We talked about this offline, and I suggested either Event or PowerSample - preferably Event because it's so core, as long as it works from Ethan's perspective.)

[eakuefner]

I'm working on a big patch to convert the Event hierarchy to classes. I don't think we need to exercise the same caution that we did for arrow functions in terms of "stuff blowing up", so I'm fine with this being a big patch, especially since people work with Events so often that making it a class should be a nice usability improvement.

[eakuefner]

Revisiting this, folks are already using classes; I think we should turn them on in the style guide.

[petrcermak]

Should we convert all classes to use proper classes?

[benshayden]

Converting at least the Event hierarchy would fix the prototype.constructor link, which would vastly simplify EventRegistry as discussed in https://codereview.chromium.org/2242733002/

grep says there are 499 instances of '.prototype = '. So that's a lot of typing. But there are only a dozen or two classes in the Event hierarchy, iirc.