But for real though, don't work for free, and don't work without a contract in place unless you're doing charity work. You've been working for them forever without them paying you. What incentive do they have to pay you now?
What motivates you to invest eighteen months of free work for a commercial start-up? Friendship? Promises of a significant reward? Genuinely interested in what drives you.
OP, please take this as a lesson. For a significant portion of my career thus far I've been in consulting/advisory positions with startups between the seed and Series A stages of funding. I have never, ever worked with a company for the first time without first getting a confirmed deposit payable towards the final invoice.
In the fast and loose world of startups, rule number one is "Can they pay me?" Employees should be paid before founders, not the other way around.
Practically speaking - engage a lawyer, send a letter and sue if the economics of that equation make sense. If they don't, cut your losses and move on with a lesson. Refer to some of the other comments here echoing the legal advice.