title: How to declare factory constructor in abstract classes?
body: <p>I want to declare, but not define a factory constructor in an abstract class.</p>
  
  <p>In my case, I want to create a method that accepts any class that implements a <code>String toJson()</code> method as well as a <code>fromJson(Map&lt;String, dynamic&gt; data)</code> factory constructor.</p>
  
  <p>Is there any way to achieve that in Dart?
  I'm looking for something like the following, which is not valid Dart code:</p>
  
  <pre class="lang-dart prettyprint-override"><code>abstract class JsonSerializable {
    factory fromJson(Map&lt;String, dynamic&gt; data);
    String toJson();
  }
  </code></pre>