Thrown by operations that have not been implemented yet.
This Error is thrown by unfinished code that hasn't yet implemented all the features it needs.
If a class is not intending to implement the feature, it should throw an UnsupportedError instead. This error is only intended for use during development.
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https://api.dartlang.org/stable/2.0.0/dart-core/UnimplementedError-class.html