Let RenderSliverFixedExtentBoxAdapter handle disappearing top items (#12919)

* fix with first part of test

* Test without reshifting

* Let sliver fixed extend list also handle disappearing top items.

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

@@ -148,6 +148,13 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
 
     for (int index = indexOf(firstChild) - 1; index >= firstIndex; --index) {
       final RenderBox child = insertAndLayoutLeadingChild(childConstraints);
+      if (child == null) {
+        // Items before the previously first child are no longer present.
+        // Reset the scroll offset to offset all items prior and up to the
+        // missing item. Let parent re-layout everything.
+        geometry = new SliverGeometry(scrollOffsetCorrection: index * itemExtent);
+        return;
+      }
       final SliverMultiBoxAdaptorParentData childParentData = child.parentData;
       childParentData.layoutOffset = indexToLayoutOffset(itemExtent, index);
       assert(childParentData.index == index);

packages/flutter/test/widgets/slivers_evil_test.dart

@@ -194,4 +194,67 @@ void main() {
     await tester.pumpAndSettle(const Duration(milliseconds: 122));
 
   });
+
+  testWidgets('Removing offscreen items above and rescrolling does not crash', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new CustomScrollView(
+        slivers: <Widget>[
+          new SliverFixedExtentList(
+            itemExtent: 100.0,
+            delegate: new SliverChildBuilderDelegate(
+              (BuildContext context, int index) {
+                return new Container(
+                  color: Colors.blue,
+                  child: new Text(index.toString()),
+                );
+              },
+              childCount: 30,
+            ),
+          ),
+        ],
+      ),
+    ));
+
+    await tester.drag(find.text('5'), const Offset(0.0, -500.0));
+    await tester.pump();
+
+    // Screen is 600px high. Moved bottom item 500px up. It's now at the top.
+    expect(tester.getTopLeft(find.widgetWithText(DecoratedBox, '5')).dy, 0.0);
+    expect(tester.getBottomLeft(find.widgetWithText(DecoratedBox, '10')).dy, 600.0);
+
+    // Stop returning the first 3 items.
+    await tester.pumpWidget(new MaterialApp(
+      home: new CustomScrollView(
+        slivers: <Widget>[
+          new SliverFixedExtentList(
+            itemExtent: 100.0,
+            delegate: new SliverChildBuilderDelegate(
+              (BuildContext context, int index) {
+                if (index > 3) {
+                  return new Container(
+                    color: Colors.blue,
+                    child: new Text(index.toString()),
+                  );
+                }
+                return null;
+              },
+              childCount: 30,
+            ),
+          ),
+        ],
+      ),
+    ));
+
+    await tester.drag(find.text('5'), const Offset(0.0, 400.0));
+    await tester.pump();
+
+    // Move up by 4 items, meaning item 1 would have been at the top but
+    // 0 through 3 no longer exist, so item 4, 3 items down, is the first one.
+    // Item 4 is also shifted to the top.
+    expect(tester.getTopLeft(find.widgetWithText(DecoratedBox, '4')).dy, 0.0);
+
+    // Because the screen is still 600px, item 9 is now visible at the bottom instead
+    // of what's supposed to be item 6 had we not re-shifted.
+    expect(tester.getBottomLeft(find.widgetWithText(DecoratedBox, '9')).dy, 600.0);
+  });
 }