Just to be clear, here is what I did.

  Dart source -> dart2js -> JavaScript (530K) 
  Dart source -> dart2js -> JavaScript (530K) -> Closure Compiler -> *error*  
  Dart source -> dart2js --minify -> JavaScript (240K)

If I import a module and don't use it, such as 'dart:utf', the resulting JavaScript output size does not go up. If I call a function in dart:utf the JavaScript size goes up. If I stop calling the function, the size goes down. If I create a function but don't call it, the size stays the same. If I call it, the size goes up. Everything seems to be working. They just release a new version a few days ago and I'm on the latest.