Worth looking into validating `js/foo` forms again the known externs set. Can probably be done by leveraging the Closure JS Parser.
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.
Definitely open to the idea of exposing this information to other tooling when we get there.
Closure warns about with DiagnosticGroups/UNDEFINED_VARIABLES enabled.