nameCache is always empty when format is systemjs

[charlag]

Hi
Thanks for the awesome plugin.
I wanted to report a thing which took me by surprise and took a while to figure out.
Basically nameCache will always be empty for systemjs format because there are no top-level identifiers, everything is wrapped inside System.register.
I'm not sure what can be done there without including all identifiers but maybe at least a warning would be a good idea?