You're right, it's no real need for it and it's unfeasible because you need to either (a) fully static type analyze everything or (b) what anonymous just explained... and they're both out of the question.
The refinements thing was a joke, in the spirit of what else can one add: it's a controlled form of monkeypatching / classboxing about which a lot of fuss was made in the Ruby community after Matz decided to add it to the language (http://timelessrepo.com/refinements-in-ruby), mainly because it's very hard to implement without creating more problems, even if you have your own interpreter to hack on (short answer: don't bother looking about it and even if you're smart enough to figure out a way to add it to a compile-to-JS language, don't waste your effort on it - they're not an anti-feature and they could make sense in languages that allow monkeypatching, like JS and Ruby, but they're too much work for too little benefit and too much possibility for bad use and unneeded complexity).
You're right, it's no real need for it and it's unfeasible because you need to either (a) fully static type analyze everything or (b) what anonymous just explained... and they're both out of the question.
The refinements thing was a joke, in the spirit of what else can one add: it's a controlled form of monkeypatching / classboxing about which a lot of fuss was made in the Ruby community after Matz decided to add it to the language (http://timelessrepo.com/refinements-in-ruby), mainly because it's very hard to implement without creating more problems, even if you have your own interpreter to hack on (short answer: don't bother looking about it and even if you're smart enough to figure out a way to add it to a compile-to-JS language, don't waste your effort on it - they're not an anti-feature and they could make sense in languages that allow monkeypatching, like JS and Ruby, but they're too much work for too little benefit and too much possibility for bad use and unneeded complexity).