[null-safety] remove mockito from scrollbar test (#64493)

More work towards #62886

packages/flutter/test/widgets/scrollbar_test.dart

@@ -7,7 +7,8 @@
 import 'package:flutter/src/physics/utils.dart' show nearEqual;
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
-import 'package:mockito/mockito.dart';
+
+import '../flutter_test_alternative.dart' show Fake;
 
 const Color _kScrollbarColor = Color(0xFF123456);
 const double _kThickness = 2.5;
@@ -39,14 +40,25 @@ ScrollbarPainter _buildPainter({
   )..update(scrollMetrics, scrollMetrics.axisDirection);
 }
 
-class _DrawRectOnceCanvas extends Mock implements Canvas { }
+class _DrawRectOnceCanvas extends Fake implements Canvas {
+  List<Rect> rects = <Rect>[];
+
+  @override
+  void drawRect(Rect rect, Paint paint) {
+    rects.add(rect);
+  }
+}
 
 void main() {
   final _DrawRectOnceCanvas testCanvas = _DrawRectOnceCanvas();
   ScrollbarPainter painter;
-  Rect captureRect() => verify(testCanvas.drawRect(captureAny, any)).captured.single as Rect;
 
-  tearDown(() => painter = null);
+  Rect captureRect() => testCanvas.rects.removeLast();
+
+  tearDown(() {
+    painter = null;
+    testCanvas.rects.clear();
+  });
 
   final ScrollMetrics defaultMetrics = FixedScrollMetrics(
     minScrollExtent: 0,