More groundwork to get `analyze` and IDE output to agree.

See: https://github.com/dart-lang/sdk/issues/25723

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

Since we removed our operator== overload, we can now allow people to put
these in collections again.

Also, turn on two more analyzer lints: avoid_empty_else and hash_and_equals.