anchor.diff:  import 'dart:async';
   import 'dart:io';
   
  +import 'package:logging/logging.dart';
  +
  +final Logger _logging = new Logger('sky_tools.device');
  +
   enum Artifact {
     FlutterCompiler,
   }
  
positive.code:   _ArtifactStore._();
  
    Future _downloadFile(String url, File file) async {
        HttpClient httpClient = new HttpClient();
        HttpClientRequest request = await httpClient.getUrl(Uri.parse(url));
        HttpClientResponse response = await request.close();
        if (response.statusCode != 200) throw new Exception(response.reasonPhrase);
        IOSink sink = file.openWrite();
        await sink.addStream(response);
        await sink.close();
    }
  
    Future<String> _getEngineRevision(String packageRoot) {
  
negative.code:     if (artifact == Artifact.FlutterCompiler) {
        File skySnapshotFile = new File(cacheDir.path + 'sky_snapshot');
        if (!await skySnapshotFile.exists()) {
          _logging.info('Downloading sky_snapshot from the cloud, one moment please...');
          String googleStorageUrl = 'https://storage.googleapis.com/mojo/sky/shell/linux-x64/${engineRevision}/sky_snapshot';
          await _downloadFile(googleStorageUrl, skySnapshotFile);
          ProcessResult result = await Process.run('chmod', ['u+x', skySnapshotFile.path]);
  