Peter Leibiger a5377647f4
[dart][dart-dio] Add support for default values (#8201)
* [dart-dio] Generate default value builders

* return `null` when there is no default value instead of `"null"`
* use built_value's `_initializeBuilder` to set default values

* [dart-dio] Fix default value formatting in model doc
2020-12-17 17:50:36 +08:00
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