Fix fencepost error

tools/factor-js

@@ -13,7 +13,7 @@ my %intersection;
 map ++$intersection{$_}, @$_ for @files;
 
 # Delete entries appearing in fewer than all files:
-$intersection{$_} < $#files && delete $intersection{$_} for keys %intersection;
+$intersection{$_} < @files && delete $intersection{$_} for keys %intersection;
 
 # And write the common definitions. Arbitrarily, we use the first file for ordering:
 open my $fh, '>', 'common.js';