I think I found a bug in fullcalendar.io version 3.6.1. When using a google calendar event source, the loading callback is not getting fired.
It looks like there are calls to this.calendar.pushLoading(); in fullcalendar.js right before the return Promise.construct statement is called in each of the extended EventSource classes. But in gcal.js for the Google Calendar event sources, this is missing. Same issue with the call to _this.calendar.popLoading(); when the response is received.
I added this in my local copy for my site and it seemed to work. It had worked in version 3.4.0, but it looks like the building of the EventSources was re-written in 3.5.0 and has been missing since then.
