add runArc method to Runtime to fetch an already running arc, if exists (#3561)
add runArc method to Runtime to fetch an already running arc, if exists (#3561)


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