I would love this feature, because I regularly have to deal with OOP code that is constructed solely of functions that are about three lines long. This often swaps method-level complexity for systemic complexity, and it drives me nuts. Having to look back through ten different files to find out how a method does what it does as well as what it does (i.e beyond what you find in your average comment) is infuriating. Being able to inline method definitions would be really helpful, if it could be done in a way that didn't cause breakages around 'this', and so on.