Create WidgetTester.ensureVisible(Finder) (#22620)

This is just a shorthand for
Scrollable.ensureVisible(tester.element(finder))

Fixes #8185

packages/flutter_test/lib/src/widget_tester.dart

@@ -677,6 +677,12 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
   SemanticsHandle ensureSemantics() {
     return binding.pipelineOwner.ensureSemantics();
   }
+
+  /// Given a widget `W` specified by [finder] and a [Scrollable] widget `S` in
+  /// its ancestry tree, this scrolls `S` so as to make `W` visible.
+  ///
+  /// Shorthand for `Scrollable.ensureVisible(tester.element(finder))`
+  Future<void> ensureVisible(Finder finder) => Scrollable.ensureVisible(element(finder));
 }
 
 typedef _TickerDisposeCallback = void Function(_TestTicker ticker);

packages/flutter_test/test/widget_tester_test.dart

@@ -616,6 +616,30 @@ void main() {
       semanticsHandle.dispose();
     });
   });
+
+  group('ensureVisible', () {
+    testWidgets('scrolls to make widget visible', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: ListView.builder(
+              itemCount: 20,
+              shrinkWrap: true,
+              itemBuilder: (BuildContext context, int i) => ListTile(title: Text('Item $i')),
+           ),
+         ),
+        ),
+      );
+
+      // Make sure widget isn't on screen
+      expect(find.text('Item 15', skipOffstage: true), findsNothing);
+
+      await tester.ensureVisible(find.text('Item 15', skipOffstage: false));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Item 15', skipOffstage: true), findsOneWidget);
+    });
+  });
 }
 
 class FakeMatcher extends AsyncMatcher {