title: gwt: No source code is available for type java.util.Calendar
body: <p>I tried to use <code>java.util.Calendar</code> in my GWT application as following:</p>
  
  <pre><code>Calendar cal = Calendar.getInstance();
  </code></pre>
  
  <p>then i got this error:</p>
  
  <blockquote>
    <p>No source code is available for type <code>java.util.Calendar</code>; did you
    forget to inherit a required module?</p>
  </blockquote>
  
  <p>anyone know what's wrong with it?</p>