I'm not there to gauge the rapport between you and your boss(es), or the turnover rate at the company, or how long it takes for them to hire somebody, or any of a myriad of details that would weigh in to my decision, so I can't really tell you what you should do. But if you have a good relationship with the people there (if you would consider them friends), I tend to agree with your desire to be above board and let them know what's going on. I don't know what your work is, but giving them a heads up gives them an opportunity to plan for your departure so they're not left in the lurch. Plus, you can help train your replacement if that's necessary.
At my last job, I gave a month's notice. But this was because my departure was extremely flexible, and I had a backlog of cases to finalize and I didn't want my colleagues to inherit a stack of partially-completed appeals with my terrible scribbled notes.
If you do decide to tell them, stress that it's a year-in-the-making, and until visas and boarding passes are in hand, it could change. Tell them you are not giving notice now (and that when the time comes, you will give them proper notice)... you just wanted to let them know that it's a possibility.