This test fails pretty frequently, no known regression point.
-FAIL trailer() test promise_test: Unhandled rejection with value: object "TypeError: undefined is not an object (evaluating 'res.trailer.then')"
+FAIL trailer() test assert_equals: expected (object) null but got (string) "Trailer header value"
Marked as flaky in <http://trac.webkit.org/r214929>
For whatever reason, this only happens on El Capitan bots now.
Closing. El Capitan expectations have been removed.