Enable stricter linter rules.

[pablojimpas]

Mostly trivial code changes made by running dart fix --apply

The new most important rules enabled are:

• prefer_final_locals: https://dart-lang.github.io/linter/lints/prefer_final_locals.html
• prefer_final_in_for_each: https://dart-lang.github.io/linter/lints/prefer_final_in_for_each.html
• avoid_dynamic_calls: https://dart-lang.github.io/linter/lints/avoid_dynamic_calls.html

All three rules take advantage of relatively recent improvements in the Dart SDK and thus "modernize" the codebase (eg. getting rid of all those var), they can potentially have a positive impact on performance as described in the reference links while also ensuring correctness long term.

I've also re-enabled library_private_types_in_public_api that is present in flutter_lints and was overridden locally.

EDIT:

• I've now promoted missing_required_param and missing_return to errors in the analyzer and also enabled the "strong-mode" to avoid weak implicit casts and implicit dynamic.
• A more cosmetic choice of preferring integer literals and using named constants to avoid copying.

[pablojimpas]

I've also enabled always_use_package_imports since there were only a couple of occurrences of relative imports, this way we enforce consistency on one style going forward.

While I was looking at the imports I've also notice that lib/flutter_map.dart was importing itself (because exports a lot of things) instead of the individual dependencies, that caused a bunch of warnings on pub.dev that lowered the package score:

[ibrierley]

Looks fine to me, and happy for it to be merged if ok with it. Thanks for the work.

[JaffaKetchup]

Mostly looks good. A couple of things to maybe change though, suggested below. Happy to approve or change suggestions if you disagree! Many thanks, it's already looking much better.

[JaffaKetchup]

In that case, I'm happy to merge as well. Many thanks for your contributions!