anchor.diff:    }
   
     /// Take a screenshot.  The image will be returned as a PNG.
  -  Future<List<int>> screenshot({ Duration timeout: _kLongTimeout }) async {
  +  Future<List<int>> screenshot({ Duration timeout }) async {
  +    timeout ??= _kLongTimeout;
       final Map<String, dynamic> result = await _peer.sendRequest('_flutter.screenshot').timeout(timeout);
       return BASE64.decode(result['screenshot']);
     }
  
positive.code:         waitForServiceExtension(),
          // We will never receive the extension event if the user does not
          // register it. If that happens time out.
          new Future<String>.delayed(const Duration(seconds: 10), () => 'timeout')
        ]);
        await whenResumed;
        _log.trace('Waiting for service extension');
  
negative.code:   static const String _kFlutterExtensionMethod = 'ext.flutter.driver';
    static const String _kSetVMTimelineFlagsMethod = '_setVMTimelineFlags';
    static const String _kGetVMTimelineMethod = '_getVMTimeline';
  
    static int _nextDriverId = 0;
  
  