Add deprecation warning to pub BS

[kevmoo]

Add a deprecation warning to when build/serve are executed – tell them to look at build_runner and friends.

See https://github.com/orgs/dart-lang/projects/6 – not ready to start on this. Adding for tracking purposes.

[kevmoo]

As of https://github.com/dart-lang/site-webdev/commit/db37ae6fb397ffb7c3ad9044e3af79be274f9471 – we have a URL: https://webdev.dartlang.org/dart-2

[kevmoo]

Will likely land as part of -dev.41