Make the page-view remembering logic actually work. (#9905)

It wasn't tested, which is why it didn't work before.

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

@@ -154,7 +154,9 @@ class _PagePosition extends ScrollPositionWithSingleContext {
     this.initialPage: 0,
     double viewportFraction: 1.0,
     ScrollPosition oldPosition,
-  }) : _viewportFraction = viewportFraction, super(
+  }) : _viewportFraction = viewportFraction,
+       _pageToUseOnStartup = initialPage.toDouble(),
+       super(
     physics: physics,
     context: context,
     initialPixels: null,
@@ -166,6 +168,7 @@ class _PagePosition extends ScrollPositionWithSingleContext {
   }
 
   final int initialPage;
+  double _pageToUseOnStartup;
 
   double get viewportFraction => _viewportFraction;
   double _viewportFraction;
@@ -198,7 +201,7 @@ class _PagePosition extends ScrollPositionWithSingleContext {
     if (pixels == null) {
       final double value = PageStorage.of(context.storageContext)?.readState(context.storageContext);
       if (value != null)
-        correctPixels(getPixelsFromPage(value));
+        _pageToUseOnStartup = value;
     }
   }
 
@@ -207,7 +210,7 @@ class _PagePosition extends ScrollPositionWithSingleContext {
     final double oldViewportDimensions = this.viewportDimension;
     final bool result = super.applyViewportDimension(viewportDimension);
     final double oldPixels = pixels;
-    final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? initialPage.toDouble() : getPageFromPixels(oldPixels, oldViewportDimensions);
+    final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? _pageToUseOnStartup : getPageFromPixels(oldPixels, oldViewportDimensions);
     final double newPixels = getPixelsFromPage(page);
     if (newPixels != oldPixels) {
       correctPixels(newPixels);

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

@@ -300,6 +300,9 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// The default implementation reads the value from the nearest [PageStorage]
   /// found from the [context]'s [ScrollContext.storageContext] property, and
   /// sets it using [correctPixels], if [pixels] is still null.
+  ///
+  /// This method is called from the constructor, so layout has not yet
+  /// occurred, and the viewport dimensions aren't yet known when it is called.
   @protected
   void restoreScrollOffset() {
     if (pixels == null) {

packages/flutter/test/widgets/page_view_test.dart

@@ -398,4 +398,63 @@ void main() {
     await tester.pump();
     expect(changeIndex, 0);
   });
+
+  testWidgets('PageView can restore page',
+      (WidgetTester tester) async {
+    final PageController controller = new PageController();
+    final PageStorageBucket bucket = new PageStorageBucket();
+    await tester.pumpWidget(
+      new PageStorage(
+        bucket: bucket,
+        child: new PageView(
+          controller: controller,
+          children: <Widget>[
+            const Placeholder(),
+            const Placeholder(),
+            const Placeholder(),
+          ],
+        ),
+      ),
+    );
+    expect(controller.page, 0);
+    controller.jumpToPage(2);
+    expect(await tester.pumpAndSettle(const Duration(minutes: 1)), 1);
+    expect(controller.page, 2);
+    await tester.pumpWidget(
+      new PageStorage(
+        bucket: bucket,
+        child: new Container(),
+      ),
+    );
+    expect(() => controller.page, throwsAssertionError);
+    await tester.pumpWidget(
+      new PageStorage(
+        bucket: bucket,
+        child: new PageView(
+          controller: controller,
+          children: <Widget>[
+            const Placeholder(),
+            const Placeholder(),
+            const Placeholder(),
+          ],
+        ),
+      ),
+    );
+    expect(controller.page, 2);
+    await tester.pumpWidget(
+      new PageStorage(
+        bucket: bucket,
+        child: new PageView(
+          key: const Key('Check it again against your list and see consistency!'),
+          controller: controller,
+          children: <Widget>[
+            const Placeholder(),
+            const Placeholder(),
+            const Placeholder(),
+          ],
+        ),
+      ),
+    );
+    expect(controller.page, 0);
+  });
 }