Class prototype property AST

[helixbass]

@GeoffreyBooth PR for class "prototype property" AST

As far as I'm aware, there's no corresponding proposed JS syntax/Babel AST for class properties that get attached to the prototype eg:

  class A
    b: 3

So this PR introduces a new node class/AST type ClassPrototypeProperty to represent these

Also contains a fix for #5204 (the invalid JS for computed class prototype properties, not my second comment in that issue). If you'd prefer I could extract the fix to a separate PR against master

[helixbass]

This is the fix for #5204

[helixbass]

If you remove the conditional logic here (so just new Access base) no tests break. But I don't feel completely confident removing it. I traced the code's origin back to #4354

[helixbass]

This mimics the properties of Babel AST types like ClassProperty/ObjectProperty

[GeoffreyBooth]

This looks fine. One thing to consider is class fields: https://github.com/tc39/proposal-class-fields

Which are currently in Stage 3 but are already shipped in Chrome and Node 12, so we’ll probably need to support them soon.

[helixbass]

Extracted the fix for #5204 to a PR against master (#5206). So once that gets merged, if you merge master into ast, I can add AST tests here for static computed class properties as well

One thing to consider is class fields

Ok so my understanding is this:

"Class fields" in JS eg

class A {
  x = 3
}

represent properties set on new class instances, so basically correspond to the Coffeescript:

class A
  constructor: ->
    @x = 3

I personally find the JS syntax weird, I guess I'm more accustomed to Python/Ruby/Coffeescript-style initialization of instances inside the constructor

So I'd assume we don't need to provide a syntax that compiles to JS class fields (since you can achieve the equivalent by initializing inside the constructor)?

Then I'm also familiar with arrow function "class fields" (I don't know the actual JS nomenclature/proposals) eg

class A {
  b = () => 3
}

And this seems to correspond to Coffeescript bound methods eg

class A
  b: => 3

But then Coffeescript's "prototype properties" eg

class A
  b: 3

are a different thing that just get set once per class (on the prototype) rather than once per instance. If there's a JS proposal for this I haven't seen it, which is why I create a new (non-Babel) AST type ClassPrototypeProperty in this PR to represent them