Make remember_scroll_position_test.dart pass (#8074)

We've decided not to store the scroll position in PageStorage because
routes now maintainState by default.

Fixes #8051

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -124,7 +124,7 @@ abstract class ViewportOffset extends ChangeNotifier {
 
   @mustCallSuper
   void debugFillDescription(List<String> description) {
-    description.add('offset: ${pixels.toStringAsFixed(1)}');
+    description.add('offset: ${pixels?.toStringAsFixed(1)}');
   }
 }
 

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

@@ -19,13 +19,15 @@ import 'sliver.dart';
 class PageController extends ScrollController {
   PageController({
     this.initialPage: 0,
-  });
+  }) {
+    assert(initialPage != null);
+  }
 
   final int initialPage;
 
   double get page {
-    final ScrollPosition position = this.position;
-    return position.pixels / position.viewportDimension;
+    final _PagePosition position = this.position;
+    return position.page;
   }
 
   Future<Null> animateToPage(int page, {
@@ -71,10 +73,14 @@ class _PagePosition extends ScrollPosition {
     state: state,
     initialPixels: null,
     oldPosition: oldPosition,
-  );
+  ) {
+    assert(initialPage != null);
+  }
 
   final int initialPage;
 
+  double get page => pixels / viewportDimension;
+
   @override
   bool applyViewportDimension(double viewportDimension) {
     final double oldViewportDimensions = this.viewportDimension;

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

@@ -12,7 +12,9 @@ import 'scroll_position.dart';
 class ScrollController {
   ScrollController({
     this.initialScrollOffset: 0.0,
-  });
+  }) {
+    assert(initialScrollOffset != null);
+  }
 
   /// The initial value to use for [offset].
   ///

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

@@ -275,7 +275,7 @@ class ScrollPosition extends ViewportOffset {
         }
         return true;
       });
-      double oldPixels = _pixels;
+      final double oldPixels = _pixels;
       _pixels = value - overScroll;
       if (_pixels != oldPixels) {
         notifyListeners();

packages/flutter/test/widgets/remember_scroll_position_test.dart

@@ -13,19 +13,20 @@ class ThePositiveNumbers extends StatelessWidget {
   final int from;
   @override
   Widget build(BuildContext context) {
-    return new ScrollableLazyList(
+    return new ListView.builder(
       itemExtent: 100.0,
-      itemBuilder: (BuildContext context, int start, int count) {
-        List<Widget> result = new List<Widget>();
-        for (int index = start; index < start + count; index += 1)
-          result.add(new Text('${index + from}', key: new ValueKey<int>(index)));
-        return result;
+      itemBuilder: (BuildContext context, int index) {
+        return new Text('${index + from}', key: new ValueKey<int>(index));
       }
     );
   }
 }
 
-Future<Null> performTest(WidgetTester tester, bool maintainState) async {
+Future<Null> performTest(WidgetTester tester) async {
+}
+
+void main() {
+  testWidgets('whether we remember our scroll position', (WidgetTester tester) async {
     GlobalKey<NavigatorState> navigatorKey = new GlobalKey<NavigatorState>();
     await tester.pumpWidget(new Navigator(
       key: navigatorKey,
@@ -34,13 +35,11 @@ Future<Null> performTest(WidgetTester tester, bool maintainState) async {
           return new MaterialPageRoute<Null>(
             settings: settings,
             builder: (_) => new Container(child: new ThePositiveNumbers(from: 0)),
-          maintainState: maintainState,
           );
         } else if (settings.name == '/second') {
           return new MaterialPageRoute<Null>(
             settings: settings,
             builder: (_) => new Container(child: new ThePositiveNumbers(from: 10000)),
-          maintainState: maintainState,
           );
         }
         return null;
@@ -59,11 +58,8 @@ Future<Null> performTest(WidgetTester tester, bool maintainState) async {
     expect(find.text('10'), findsNothing);
     expect(find.text('100'), findsNothing);
 
-  Completer<Null> completer = new Completer<Null>();
-  tester.state<ScrollableState>(find.byType(Scrollable)).scrollTo(1000.0).whenComplete(completer.complete);
-  expect(completer.isCompleted, isFalse);
+    tester.state<Scrollable2State>(find.byType(Scrollable2)).position.jumpTo(1000.0);
     await tester.pump(const Duration(seconds: 1));
-  expect(completer.isCompleted, isTrue);
 
     // we're 600 pixels high, each item is 100 pixels high, scroll position is
     // 1000, so we should have exactly 6 items, 10..15.
@@ -100,8 +96,8 @@ Future<Null> performTest(WidgetTester tester, bool maintainState) async {
 
     // Ensure we don't clamp the scroll offset even during the navigation.
     // https://github.com/flutter/flutter/issues/4883
-  LazyListViewport viewport = tester.firstWidget(find.byType(LazyListViewport));
-  expect(viewport.scrollOffset, equals(1000.0));
+    Scrollable2State state = tester.state(find.byType(Scrollable2).first);
+    expect(state.position.pixels, equals(1000.0));
 
     await tester.pump(const Duration(seconds: 1));
 
@@ -119,11 +115,5 @@ Future<Null> performTest(WidgetTester tester, bool maintainState) async {
     expect(find.text('15'), findsOneWidget);
     expect(find.text('16'), findsNothing);
     expect(find.text('100'), findsNothing);
-}
-
-void main() {
-  testWidgets('whether we remember our scroll position', (WidgetTester tester) async {
-    await performTest(tester, true);
-    await performTest(tester, false);
   });
 }