NNBD and more darty api #86
Description
In dart, the convention is camelCase, final fields, and now, nnbd. The package needs an great migration. I did it at my github, but it is an humongous breaking change and contains 3 packages that i made and didnt publish to pub.dev. I tried to only migrate to nnbd but because the fields are set after object initialization, it proved to be rather chalenging. So as it was becoming an big breaking change with big consequences i ended up doing it my way.