Before this, if a new global was added to JS code (module toplevel,
function bodies are not considered) it would blow up when attempting
to resolve variable refs & uses.
Make the toplevel scope a defaultdict such that if e.g. a "foo" is
looked up in a module but not a local variable of that module it's
assumed to be an UnknownNS object (from which further UnknownNS can be
deref'd).
Xavier Morel
e8dc72fd69