This command uses package:test to run Dart tests with sky_shell.  For this to
work, we need https://github.com/dart-lang/test/tree/hacky-loader-hook to land.
We're also not smart enough to find sky_shell ourselves yet. Instead, we take
the path as input using an environment variable. Eventually, we'll be able to
get the sky_shell executable from package:sky_engine, but we don't yet ship
that executable.