CREATE DOMAIN json vs built-in json

I've noticed our plv8 regression test now fails.  It has CREATE DOMAIN
json AS text ... and validates text via v8's JSON.parse(), which was
working before introducing built-in json type.  The test itself can be
solved simply by creating schema, but my surprise is that we allow a
domain whose name is the same as other base type.  Is it intentional?

Thanks,
-- 
Hitoshi Harada