Lazy-load widgets in jupyterlab

js/src/jupyterlab-plugin.js

@@ -1,15 +1,28 @@
-var jupyter_threejs = require('./index');
-
 var base = require('@jupyter-widgets/base');
+var version = require('./version');
 
 module.exports = {
     id: 'jupyter.extensions.jupyter-threejs',
     requires: [base.IJupyterWidgetRegistry],
     activate: function(app, widgets) {
         widgets.registerWidget({
             name: 'jupyter-threejs',
-            version: jupyter_threejs.version,
-            exports: jupyter_threejs
+            version: version.version,
+            exports: function(){
+                return new Promise(function(resolve, reject){
+                    require.ensure(
+                        ['./index'],
+                        function(require) {
+                            resolve(require('./index'));
+                        },
+                        function(err) {
+                            console.error(err);
+                            reject(err);
+                        },
+                        'jupyter-threejs'
+                    );
+                });
+            }
         });
     },
     autoStart: true