Include extension types in 'implementers' list

[srawlins]

Fixes #3656

I'll get to the punchline quickly: The gist of this fix is a single line in package_graph.dart: _addToImplementers(library.extensionTypes);. The rest is mostly test infra, tech debt cleanup, and yak-shaving:

• There were no template tests that validate what goes into the implementers section, so I added class_test.dart to introduce such tests.
• addImplementer(), which must now also consider extension types, did so many as InheritingContainer calls that I decided to add some helpers to do all of this casting as part of this or that class's responsibility, instead of the responsibility of addImplementor(). So mixedInTypes now has sibling mixedInElements, and publicInterfaces has publicInterfaceElements, etc. I found most of the callers concerned themselves with the elements, so I changed most callers to use mixedInElements. Same with implementors etc.
• I made mixedInTypes @visibleForTesting since it is almost entirely replaced by mixedInElements. But it was still used in templates.runtime_renderers.dart until I corrected the codegen to account for a PropertyAccessorElement's variable.

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[kallentu]

Could we name this something slightly different from line 545 (or the opposite)? My eyes are glazing over see them because they're so similar haha.

[kallentu]

And also keep implementor vs implementer spelling consistent.

[srawlins]

I've renamed the lower one immediateImplementors.

It looks like everything in this file is implementor; the display name in the HTML is "implementer", which I think is more correct. But to change everything in this file would make the diff even bigger. I'd like to address that in a follow-up.

[kallentu]

For all the tests, maybe group them like test_class_extends test_class_implements test_mixin_constraintsSuperclass?

[srawlins]

Done

[kallentu]

This isn't? That's surprising haha.

[srawlins]

Yeah I wrote the test because I assumed it worked. :/