anchor.diff:  
       RegExp timeRegExp = new RegExp(r'^\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{3}', multiLine: true);
       Match timeMatch = timeRegExp.firstMatch(output);
  -    return timeMatch[0];
  +    return timeMatch?.group(0);
     }
   
     Future<String> stopTracing(AndroidApk apk, { String outPath }) async {
  
positive.code:     ]));
    }
  
    // Return the most recent timestamp in the Android log. The format can be
    // passed to logcat's -T option.
    String get lastLogcatTimestamp {
      String output = runCheckedSync(adbCommandForDevice(<String>[
        '-s', id, 'logcat', '-v', 'time', '-t', '1'
  
negative.code:     String tracePath = null;
      bool isComplete = false;
      while (!isComplete) {
        List<String> args = <String>['-s', id, 'logcat', '-d'];
        if (beforeStop != null)
          args.addAll(<String>['-T', beforeStop]);
        String logs = runCheckedSync(adbCommandForDevice(args));
        Match fileMatch = traceRegExp.firstMatch(logs);
        if (fileMatch != null && fileMatch[1] != null) {
          tracePath = fileMatch[1];
  