Fixes type of the `tags` variable. (#2853)
Fixes type of the `tags` variable. (#2853)

It is now always a Set (used to be either a Set or an array, which have
different forEach methods, which was causing an error).
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs