Simplify InheritingContainer.allModelElements and _inheritedElements

[srawlins]

I could not wrap my head around the interactions between InheritingContainer._allModelElements, which seemed to fill a cache, then do work, then clear the cache, and InheritingContainer._inheritedElements. _inheritedElements and _inheritedElementsCache seemed like a clear instance of a late final field before null safety (before late), but then the cache invalidation in _allModelElements kind of nullifies that theory...

Anyways, it can all be simpler:

• _allModelElements can just be the list.
• _inheritedElementsCache is no more; _inheritedElements is now a late final.
• I cannot fathom why inheritanceChainElements was initialized in the for loop... so its now initialized outside the for loop. 🤷
• Iterating over someMap.keys and immediately calling back into the map and null-asserting the result is too much work. We can instead iterate over the entries (hopefully performant...) and destructure the result.
• Add more comments for the next archaeologist.

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[kallentu]

    if (element case ClassElement classElement
        when classElement.isDartCoreObject) {
      return const <ExecutableElement>[];
    }

Maybe? Instead of casting.

[srawlins]

I like it

[kallentu]

Which is inheritedMap2? Is this comment outdated or am I missing something?
Or does this just mean inheritanceMap and "the one provided" == inheritedElement?
Maybe fix the comments to be a tad more clear.

[srawlins]

Oops, wrong name. I am referring to the getInheritedMap2 method. Updated.

[kallentu]

Also these three maps above are named so similarly. Can we add a comment on what the difference is between them?

[srawlins]

Ugh, the doc comment on inheritanceChain only describes what it is not ("Not the same as [superChain] as it may include mixins."). OK so I've updated that comment. And some more. And then introduced some comments above these declarations.

I cannot think of better names; they're already long and I think making them longer would hurt readability. But I'm open to new name ideas.

FWIW I don't know why this algorithm was chosen for _inheritedElements, but I'm strongly preferring to leave it alone; it is likely depended on by Flutter. It might be documented in tests.