skyx files are zips, so they can have anything at the start. Having
a shebang line at the start makes it easier to run skyx files in a mojo
environment.
This command will produce an flx package. Currently, this command doesn't work
because we don't have the Flutter compiler downloaded from Google storage yet.
A future patch will make that happen.
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.
