Upgrade Dart2 template to advertised best-practices, plenty of additions too.

[noordawod]

This upgrades the Dart2 template to advertised best-practices by the Dart team and most importantly, use {{{ }}} to reference variables instead of {{ }} since the latter is for escaping values. It happened to me that occasionally the generated code would include " and &#XX; escape characters for some reason or another.

Few additions were added too:

• Allow users to decide whether an invalid enum throws an error or not -- handy since in the old case, when an API changes, outdated apps would crash.
• Added growable to most calls that returns a List. Defaults to not allow growing a returned list.
• Uses functional calls where possible (=>).
• Uses final variables where possible.
• Uses singleton instances for type transformers, lazily-instantiated.
• Added comments where needed.

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.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. 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
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

@ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09) @athornz (2019/12) @amondnet (2019/12) @wing328

[agilob]

To summarize my comments above, dart-lang says to prefer single quotes https://github.com/dart-lang/pedantic/#stricter-than-effective-dart.

[noordawod]

Thanks @agilob for the review.
I have answered your question regarding the use of double-quotes. If you're not convinced, I can roll back to using single quotes, no problem at all.
The rest of the notes you've raised, I answered or dealt with.

What do you think?

[noordawod]

@agilob

Running ./bin/generate-samples.sh and ./bin/utils/ensure-up-to-date causes many files unrelated to Dart2 to change. Kind of a mess to prepare a pr for a specific generator/template and include other files too. This is partly why the CI is failing.

Am I doing something wrong?

[agilob]

@agilob

Running ./bin/generate-samples.sh and ./bin/utils/ensure-up-to-date causes many files unrelated to Dart2 to change. Kind of a mess to prepare a pr for a specific generator/template and include other files too. This is partly why the CI is failing.

Am I doing something wrong?

1. Make sure you're using latest java package too, I often make the mistake and forget to recompile maven project! mvn clean package -DskipTests=true && ./bin/generate-samples.sh

2. ./bin/generate-samples.sh accepts input like bin/configs/dart-petstore_client_lib.yaml

mvn clean package -DskipTests=true && ./bin/generate-samples.sh bin/configs/dart-petstore_client_lib.yaml
mvn clean package -DskipTests=true && ./bin/generate-samples.sh bin/configs/dart-openapi.yaml this one is removed in my dart1 removal PR!

you don't need to run all configs, just select the ones your changes affect

[agilob]

Calm down with changes now, this is massive PR ;)

[noordawod]

/me Calming down...

[wing328]

@noordawod thanks for the PR. Can you please resolve the merge conflicts when you've time?

[noordawod]

@wing328 @agilob

What you guys think about the pr? Any other comments?

[wing328]

@noordawod let's go ahead with this PR. Thanks for the contribution.

[noordawod]

It's been a pleasure guys, thank you for merging it!