[DART2]: fix Map<String, nativeType> fromJson

[d3v-cl]

@ircecho @swipesight @jaumard

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Description of the PR

Maps with native types could not be deserialized.
_JsonMap is not a Map<String, String>

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

cc @ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09)

[d3v-cl]

Hi @wing328

I hope this is fixed now.

[ircecho]

I checked the PR and it looks good to me.

[wing328]

@ircecho thanks for reviewing the change.

@d3v-cl thanks for the PR.