The issue was noticed in Firefox when user with login "watch"
was registered in the database. And it was caused by Object
in Firefox having "watch" method.

Solved by checking for whether lookup object does have own
property before using it as a map key.

To test the issue simply create a user with login "watch",
open the phabricator site in Firefox and try to assign any
maniphest task to this user. You'll see exception being
printed to the javascript console (in my case it's printed
to firebug console).