routing assertions do not support constraints

# in config/routes.rb
match 'profile', :to => 'users#profile', :constraints => {:subdomain => /.+/ }

# in test/functional/profile_routing_test.rb
assert_generates "/profile", :controller => :users, :action => :profile

# result of running test
undefined method `subdomain' for #<Rack::Request:0x102ed9268>

It appears that routing assertions have not been updated along with changes to the router, and do not support constraints that interact with the request. Note that the error states Rack::Request, and not ActionDispatch::Request, which handles constraints like subdomain.