[dart] Use raw strings for json/header parameter names

[kuhnroyal]

This should fix all problems related to unescaped characters in strings that are assumed to be raw - e.g. json['$special[property.name]'] -> json[r'$special[property.name]'].

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CC @ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @agilob

[wing328]

CircleCI failure not related to this change.