function makeManifestText(timeline, testAttrs, dur, startTime) {
        const start = '<' + type + ' ' + extraAttrs + ' ' + testAttrs + '>';
        const end = '</' + type + '>';
        const lines = [].concat(start, extraChildren, timeline, end);
        return Dash.makeSimpleManifestText(lines, dur, startTime);
      }
@param {!Array<string>} timeline
@param {string} testAttrs
@param {number=} dur
@param {number=} startTime
@return {string}