It'd be helpful to allow `mach watch` for non-artifact builds, so you can gain its benefits when working with such a build (for the subset of changes that `mach watch` supports, i.e. frontend changes that don't require recompilation). Presumably we'd want to at least warn users that `mach watch` won't handle recompilation when running the command with such a build. Even better might be to avoid warning users until they make such a change. At first glance, this isn't as simple as removing the `if not conditions.is_artifact_build(self)` conditional in mach_commands.py, since doing so causes this error when running the command: IOError: [Errno 2] No such file or directory: u'…/obj-x86_64-apple-darwin15.6.0/faster/unified_install_dist_bin'