Ensure barrier is communicated back to runtime when setting variable. (#1804)
Ensure barrier is communicated back to runtime when setting variable. (#1804)

This allows barriers to be communicated back to the proxy and thus
removed, which in turn allows communication via variable between
multiple simultaneous views of an arc.
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs