       _mediaKeySession = _mediaKeys.createSession('persistent-license');
            _mediaKeySession.addEventListener('message', onMessage);
            _mediaKeySession.closed.then(onClosed);
            return _mediaKeySession.load(event.data.sessionId);
        }).then(function(success) {
            assertions.push({actual: success, expected: true, message: "Expect load session to succeed"});
            if (!success) throw new DOMException();
            return testmediasource(config);
        }).the