title: Converting object to an encodable object failed
body: <p>I am getting the following error:</p>
  
  <pre><code>Converting object to an encodable object failed: Instance of 'Patient'
  #0      _JsonStringifier.writeObject (dart:convert/json.dart:674)
  #1      _JsonStringifier.writeList (dart:convert/json.dart:724)
  #2      _JsonStringifier.writeJsonValue (dart:convert/json.dart:706)
  #3      _JsonStringifier.writeObject (dart:convert/json.dart:664)
  #4      _JsonStringStringifier.printOn (dart:convert/json.dart:873)
  #5      _JsonStringStringifier.stringify (dart:convert/json.dart:855)
  #6      JsonEncoder.convert (dart:convert/json.dart:256)
  #7      JsonCodec.encode (dart:convert/json.dart:155)
  #8      Persistence.saveLatestPatients (/Users/dean/Library/Developer/CoreSimulator/Devices/570CC18D-95BF-4062-8523-9C78E106D0CF/data/Containers/Data/Application/70CAEFAA-4AE3-4CBF-A85F-39161E472C83/tmp/flutter_prototypev6jYbr/flutter_prototype/lib/utils/persistence.dart:32:23)
  &lt;asynchronous suspension&gt;
  #9      _HomeScreenState.fetchData.&lt;anonymous closure&gt; (/Users/dean/Librar&lt;…&gt;
  </code></pre>
  
  <p>My 'Patient' class:</p>
  
  <pre><code>import 'package:simple_moment/simple_moment.dart';
  
  class Patient {
    String guid;
    String _name;
    String _surname;
    DateTime _updated;
  
    Patient(String guid) {
      this.guid = guid;
    }
  
    String get name =&gt; _name;
    set name(v) =&gt; _name = v;
  
    String get surname =&gt; _surname;
    set surname(v) =&gt; _surname = v;
  
    DateTime get updated =&gt; _updated;
    set updated(v) =&gt; _updated = v;
  
    // Helper functions
  
    String getFullName() =&gt; '$_name $_surname';
  
    String getRelativeLastUpdated() {
      var moment = new Moment.now();
      return moment.from(_updated);
    }
  
  }
  </code></pre>