Merge pull request #36 from dart-lang/devoncarew-patch-1

Update pubspec.yaml

Prepare to publish stable null safety (#28)

Bump SDK constraints for pub (#25)

Use a 2.12.0 lower bound since pub does not understand allowed
experiments for earlier versions.

Use a 3.0.0 upper bound to avoid a warning in pub and to give some
flexibility in publishing for stable.

allow the 2.12 prerelease sdks (#24)

Prepare for the 2.11 dev SDKs (#23)

Bump the upper bound to allow 2.10 stable and 2.11.0 dev SDK versions.

CI: Test on dev branch (#19)

Merge in features from the quiver clock class.