Breaking: Only add .implements/.accessibility/.decorators if "truthy"

[JamesHenry]

This changes the way implements, accessibility and decorators are applied.

When running babylon AST comparison tests, I noticed that babylon only applies the above properties if they are "truthy" (not exactly the right way to describe it but hopefully it's clear what I mean).

For example, if there is at least one decorator on a node, there will be a decorators array in the AST, otherwise, there will be no decorators property at all.

Currently in typescript-eslint-parser, we usually apply an empty array for the decorators in that case.

Before reviewing and merging this PR, I would first like to confirm that this is indeed the desired behaviour, @Andy-MS?

I am personally pretty ambivalent about which makes more sense and would be keen to hear your thoughts on why you opted from leaving them out of the AST vs empty array/null value.

cc @hzoo @soda0289

[eslintbot]

LGTM

[eslintbot]

Thanks for the pull request, @JamesHenry! I took a look to make sure it's ready for merging and found some changes are needed:

• Pull requests with code require an issue to be mentioned at the end of the commit summary, such as (fixes #1234). Please update the commit summary with an issue (file a new issue if one doesn't already exist).

Can you please update the pull request to address these?

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[ghost]

This isn't a big issue for me either; I was following the behavior of the flow plugin, which only adds plugin-specific properties (such as .typeAnnotation) if plugin-specific syntax is present. I don't know if this is actually important anywhere in the babel toolchain, but it does make the JSON representation cleaner.

[JamesHenry]

Ok, thanks a lot for the feedback! I can't think of any strong reason not to do that (if downstream tools like prettier or the typescript plugin are assuming the properties exist that will be easy enough to fix) I'll request a review from @soda0289, and he can voice any concerns he may have if any.

[soda0289]

LGTM. Might have some issues if plugins expected those properties but I don't think any do.