                 assert_equals(mediaSource.activeSourceBuffers.length, 2,
                                "activeSourceBuffers list updated when tracks are loaded");
                              assert_equals(mediaSource.activeSourceBuffers[0], mediaSource.sourceBuffers[0],
                                "first active SourceBuffer matches first SourceBuffer");
                              assert_equals(mediaSource.activeSourceBuffers[1], mediaSource.sourceBuffers[1],
                                "seco