And does it work?
I made a test using

{ module: "calendar", position: "top_left", maximumEntries: 8, config: { calendars: [ { symbol: "calendar", url: "webcal://www.calendarlabs.com/templates/ical/US-Holidays.ics" }, { symbol: "bell", url: "https://www.calendarlabs.com/ical-calendar/ics/75/UK_Holidays.ics", maximumEntries: 4 }, { symbol: "home", url: "https://www.calendarlabs.com/ical-calendar/ics/52/Islam_Holidays.ics" } ], } },

and the symbols look OK:
0_1508900216758_MM.png