Writes all of the recorded timeline data to a file.
Source
Future<Null> writeTimelineToFile(String traceName,
{String destinationDirectory, bool pretty: false}) async {
destinationDirectory ??= testOutputsDirectory;
await fs.directory(destinationDirectory).create(recursive: true);
File file = fs.file(path.join(destinationDirectory, '$traceName.timeline.json'));
await file.writeAsString(_encodeJson(_timeline.json, pretty));
}