title: limit T in a generic class to types that implement some interface
body: <p>In dart consider the following</p>
  
  <pre><code>class Question&lt;T&gt;{
    String body;
    T answer;
  }
  
  //an interface
  class IAnswer{
    String Value()=&gt;this.toString();  
  }
  </code></pre>
  
  <p>I need T to be limited to types the for example implement the interface IAnswer, is that possible without having to check if T is IAnswer and throw and exception in the Question constructor?</p>