Dart2 template linting

[noordawod]

There have been many changes lately for the Dart linter, particularly the pedantic linting has been phased out in favor of flutter_lints. For upgrade details: https://github.com/dart-lang/lints#migrating-from-packagepedantic

Anyway, when I regenerated the source code using the current Dart template and using the linting rules that comes with flutter_lints, I got many errors. In addition, I personally use a stricter linter rules and so I also added few ignore to silence the linter in such cases.

In addition, there's no more need for implicit casting in the resulting source code and the analysis_options.yaml can practically be removed now.

All in all, the generated code is much better now and will run in both pedantic and the new flutter_lints.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @wing328 @jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

[noordawod]

Anyone wants to take a look?
@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @wing328

[noordawod]

At last, green! 🤟

[noordawod]

Code is updated again with master, would be nice to have it reviewed and approved?
@kuhnroyal @wing328 @ahmednfwela

[noordawod]

@agilob updated the code per your notes, ball's at your end. Thanks!

[noordawod]

@agilob you think it's possible to merge this pr this week?

[agilob]

@wing328 can we get this in pls?

[noordawod]

@wing328 maybe start the week with a positive merge vibes? :)

[agilob]

I would like this merged as it will conflict with my dart2 NNBD branch

[agilob]

Any comments from @kuhnroyal?

[wing328]

Just merged. Sorry for the delay.