Fix anno deserialization when class field is not first (#2501)

Update `findTypeHints` to allow for the "class" field in JSON objects to
appear anywhere in the object. This used to rely on the field being the
very first in the object, which is easily violated when reading JSON
data generated externally, since an object's order of fields is
unspecified and can be arbitrarily scrambled.

Fixes #2497.