Except that when JavaScript is the target code size does matter because it has to come up over the wire no? That's why something like Dart advertises the fact that it does tree shaking. (Elm may aim for the same efficiency who knows?)
You can do tree shaking in JavaScript with tools like Google Closure Compiler and uglifyjs (a bit more limited). Any language which compiles to JavaScript aught to be able to make use of these tools.
