* [dart-dio+time_machine] Add missing import, serializer
* [dart-dio] Remove bad import from `http` package
* [dart-dio] Use raw strings for those that contain variable name.
This should eliminate potential issue with variables like `$ref`.
* [dart-dio] Use `_path` instead of `path` to avoid potential conflict with op params.
See 1bec0b47b1/Kubernetes.json, there are a few with `{path}` param.
* [dart-dio+time_machine] Use `OffsetXxx` classes for date time values
