Related: acornjs/acorn#541

The only point of this method besides updating the map of exported symbol
names was to complain if there were duplicate export names, but we
override the raiseRecoverable method with noopRaiseRecoverable anyway, so
there's really no need to override checkExport.

The Reify parser is more like the "loose" Acorn parser in that it doesn't
care about enforcing expectations as long they don't affect the results of
the parsing. So a loop that serves only to call this.unexpected is a loop
that we can do without.