Update render_visibility_detector_test.dart (google#400)

Author: devoncarew

packages/visibility_detector/test/render_visibility_detector_test.dart

@@ -7,7 +7,6 @@
 import 'dart:ui';
 
 import 'package:flutter/rendering.dart';
-import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:visibility_detector/src/render_visibility_detector.dart';
 import 'package:visibility_detector/visibility_detector.dart';