Fix serialization of models from js -> kernel

IPython/html/static/widgets/js/types.js

@@ -30,6 +30,8 b' define(['
                     return Promise.resolve(value);
                 }
             },
+            /* We don't need a serializer since models automatically serialize to their UUIDs */
+            /* courtesy of the model toJSON */
         },
     }
 });

IPython/html/static/widgets/js/widget.js

@@ -440,6 +440,14 b' define(["widgets/js/manager",'
             }, this);
 
         },
+
+        toJSON: function(options) {
+            /**
+             * Serialize the model.  See the types.js deserialization function
+             * and the kernel-side serializer/deserializer
+             */
+            return "IPY_MODEL_"+this.id;
+        }
     });
     widgetmanager.WidgetManager.register_widget_model('WidgetModel', WidgetModel);