Deploy Scrollable2.ensureVisible (#7951)

Also, switch Stepper over to using sliver-based scrolling.

packages/flutter/lib/src/widgets/focus.dart

@@ -193,6 +193,7 @@ class Focus extends StatefulWidget {
     if (focusScope != null) {
       focusScope.focusState._setFocusedWidget(key);
       Scrollable.ensureVisible(focusedContext);
+      Scrollable2.ensureVisible(focusedContext);
     }
   }
 
@@ -360,6 +361,7 @@ class _FocusState extends State<Focus> {
     if (focusedContext == null)
       return;
     Scrollable.ensureVisible(focusedContext);
+    Scrollable2.ensureVisible(focusedContext);
   }
 
   @override

packages/flutter_driver/lib/src/extension.dart

@@ -249,6 +249,7 @@ class _FlutterDriverExtension {
     ScrollIntoView scrollIntoViewCommand = command;
     Finder target = await _waitForElement(_createFinder(scrollIntoViewCommand.finder));
     await Scrollable.ensureVisible(target.evaluate().single, duration: const Duration(milliseconds: 100));
+    await Scrollable2.ensureVisible(target.evaluate().single, duration: const Duration(milliseconds: 100));
     return new ScrollResult();
   }