lang/spidermonkey fails to build unless devel/readline is installed. devel/readline is not specified as something spidermonkey60 does depend on, and readline is part of the system with FreeBSD 12 anyway.
System: FreeBSD lightning 12.1-STABLE FreeBSD 12.1-STABLE r357702 LIGHTNING amd64
(without devel/readline installed)
> checking for readline in -lreadline... no
> configure: error: No system readline library found.
With devel/readline installed, spidermonkey60 builds and installs as normal.