Latest Esprima beta changes

[sam-lord]

This update is a pre-requisite for adding arrow functions.

Unfortunately as I'm not completely familiar with the code / intentions behind all of the changes in Esprima, I may have gone about this in the wrong way (in particular I do recall reading that Esprima.Ast.List is not supposed to be exposed?).

I'd be happy to take any comments on board. Once this is in, the change to add arrow functions is very minor.

[lahma]

Looks like a good cleanup, thanks for taking the time! Could you check my comments about passing Esprima's List as ref due to it being a struct, otherwise it gets copied every time.

[Happypig375]

I think using the in modifier instead of ref would be better since it guarantees the list cannot be reassigned.

[lahma]

But as Esprima list is not read-only I believe a defensive copy would be made.

[sam-lord]

I agree with @lahma. I've just been reading this blog post to get a better grasp on the in modifier in C#: https://blogs.msdn.microsoft.com/seteplia/2018/03/07/the-in-modifier-and-the-readonly-structs-in-c/ - which concludes that a defensive copy will be made for structs which are not read-only.

I'll go ahead and update this PR with ref keywords.

[sam-lord]

Use of in modifier

[sam-lord]

Is there anything stopping this from being merged in?

[lahma]

Sorry to keep you waiting. @sebastienros makes the final call and I believe he's been a bit under the weather, hopefully back soon to review.