To be filled in by a later CL. I deployed a test version to
App Engine to work on setting the values, so there are
a few records that have this field set already.

That field is breaking the live version, so I have pushed
a new copy with this 1-line change to the live version

I assumed that appengine/datastore was like every other
marshaling and unmarshaling package we have in Go
(for example, encoding/gob, encoding/json, encoding/xml,
and protobuf) and that if it loaded an unknown field it would
just ignore it. Apparently not. Sorry.

R=dsymonds
TBR=dsymonds
CC=golang-dev
https://golang.org/cl/6454064