Avoid uses of ES6 Set, use Array instead

[ajafff]

Fixes: #28918

/cc @Andy-MS

[arktronic]

This looks like a step in the right direction for fixing #28918, but I believe more needs to be done for that issue. For example, inspectValue.ts also uses String.prototype.startsWith, which is another function introduced in ES6. I'm unsure of how many other ES5 incompatibilities exist in this and/or in other files.

[NN---]

Why just not use core-js or any other polyfill ?

[arktronic]

Given the fact that functions like ts.startsWith exist in the codebase, it would appear that TypeScript is intended to run in a pre-ES6 environment without polyfills. Official guidance on this from project maintainers would be helpful, of course.

[j-oliveras]

I opened #28932 to solve the String.prototype.startsWith and some others.

[weswigham]

We probably used Set because we assumed we had self-polyfilled it like we do Map - it's probably best to write out the polyfill instead.

[ajafff]

it's probably best to write out the polyfill instead.

What's the benefit of that? There are still other ES6 features without a polyfill that could easily be used by accident (see #28933).
Also note that Set doesn't give any additional performance and isn't easier to use in these cases.

[weswigham]

What's the benefit of that?

Using the platform Set when it's available, mostly (which normally has better memory characteristics, at least). Like what we do with Map would be ideal.