BUG 4.1-RC1: Model instance with proxy causes exception when Ext.required by itself

[JamaSoftware-Sencha]

Example model:

Ext.define('jx.model.Artifact', {
    extend: 'Ext.data.Model',
    requires: [
        'Ext.data.proxy.Ajax',
        'Ext.data.reader.Json'
    ],
    fields: [{
        name: 'id',
        defaultValue: null,
        mapping: 'id'
    }, {
        name: 'version',
        defaultValue: null,
        mapping: 'version'
    }],
    
    proxy: {
        type: 'rest',
        url: '/artifacts',
        reader: {
            type: 'json'
        }
    }
});

If I have a page (in this case a unit test) that includes this model synchronously (although I don't believe the synchronicity should matter):

Ext.syncRequire('jx.model.Artifact');

I get an exception in Ext.data.Proxy, line 225:

if (!(writer instanceof Ext.data.writer.Writer)) {

Exception:

Ext.data.writer is undefined

Possible workaround is to require "Ext.data.writer.Writer" in my model instance, but really "Ext.data.Proxy" should be requiring this class since it needs it.

[mitchellsimoens]

Ext.data.writer.Writer should be added to the requires array.