support sending `eventInit` options to `_fire` (rare: mainly to allow bubbling)
support sending `eventInit` options to `_fire` (rare: mainly to allow bubbling)

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs