dillon1000/react

Remove Agent addBridge: there can only be one bridge, add in constructor

Let's make impossible states truly impossible, and fix Flow types, too.

All three usages of Agent called addBridge right after constructing it.
Agent has one field `_bridge` which is force-typed as not-null despite
there's a temporary zone between the constructor end and addBridge start
where `_bridge` is null.