externs checking for js/foo
Worth looking into validating `js/foo` forms again the known externs set. Can probably be done by leveraging the Closure JS Parser.
Closure warns about with DiagnosticGroups/UNDEFINED_VARIABLES enabled.
Definitely open to the idea of exposing this information to other tooling when we get there.
Would you consider making the results of parsing available to tooling (e.g. in cljs.env/compiler)? I would use this to add support for autocompletion of js/ forms to CIDER.