Add a --version flag

[natebosch]

[jakemac53]

What version would it print? The build_runner version?

[natebosch]

Possibly the version of build_runner and build_runner core?

Maybe even look at the pubspec.lock and print versions for all build_* packages?

I'd mostly just expect that binaries support --version for consistency.

[jakemac53]

Huh, I guess even pub/dartdevc/dart2js all support that. What happens if you pass it with any other arguments?

[natebosch]

What happens if you pass it with any other arguments?

I think usually they're ignored, version is printed and it exits.

[jakemac53]

huh, so the tools don't agree on what to do in the presence of other args:

dart vm: ignores the --version flag and runs the provided script dart2js/dartdevc/dartanalyzer: just prints the version and ignores other args

We should probably just do what everything but the vm does, and ignore other args.

[davidmorgan]

Closing as stale.