update ci; prep for publishing (#30)

Prepare to publish for stable null safety (#25)

Bump SDK constraints for pub (#22)

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 (#21)

Prepare for the 2.11 dev SDKs (#20)

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

fix changelog entries...

Update Travis-CI to test on dev (#18)

Remove unnecessary dependency override (#12)

Support source maps with multiple target files.