Drop default value when unnecessary.

[noordawod]

The generated Dart2 code included default values when unnecessary, this pr removes them.

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@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[noordawod]

The failed tests are unrelated to the changes in the Dart2 template, most are Python-related but one is scala-akka-related…

[noordawod]

Wondering when will this pr get merged, @agilob?

[ahmednfwela]

@wing328