From "http://leafletjs.com/reference.html#featuregroup"
L.FeatureGroup
Extended LayerGroup that also has mouse events (propagated from members
of the group) and a shared bindPopup method. Implements ILayer
interface.
From the comments, subdomains can be a string or an array of strings. We
can't make any strong type inference from the interface.
Quote:
"Subdomains of the tile service. Can be passed in the form of one string (where
each letter is a subdomain name) or an array of strings."
