Lint Dart code (#986)

pub.dev awards a score for each package, and we lose a lot of points
because of failing lints:
https://pub.dev/packages/icu4x/versions/2.0.0-dev.0/score

Bump diplomat-runtime to 0.13.1

Bump to 0.8.3

Fix runtime

Bump versions (#875)

Patch diplomat-runtime to not be a breaking change

diplomat_core v0.8.1