title: Which files are generated when executing `dart2js`? and why?
body: <p>dart2js probably generates <code>.js.map</code>, <code>.js.deps</code> and <code>.precompiled.js</code>.</p>
  
  <p>What's the purpose of them? and I donno why <code>dart2js</code> doens't remove them after finishing compile.</p>