Add assert for negative child count in ListView.builder (#45506)

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

@@ -81,6 +81,7 @@ abstract class ScrollView extends StatelessWidget {
        assert(!shrinkWrap || center == null),
        assert(anchor != null),
        assert(anchor >= 0.0 && anchor <= 1.0),
+       assert(semanticChildCount == null || semanticChildCount >= 0),
        primary = primary ?? controller == null && identical(scrollDirection, Axis.vertical),
        physics = physics ?? (primary == true || (primary == null && controller == null && identical(scrollDirection, Axis.vertical)) ? const AlwaysScrollableScrollPhysics() : null),
        super(key: key);
@@ -943,7 +944,9 @@ class ListView extends BoxScrollView {
     double cacheExtent,
     int semanticChildCount,
     DragStartBehavior dragStartBehavior = DragStartBehavior.start,
-  }) : childrenDelegate = SliverChildBuilderDelegate(
+  }) : assert(itemCount == null || itemCount >= 0),
+       assert(semanticChildCount == null || semanticChildCount <= itemCount),
+       childrenDelegate = SliverChildBuilderDelegate(
          itemBuilder,
          childCount: itemCount,
          addAutomaticKeepAlives: addAutomaticKeepAlives,

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

@@ -88,6 +88,7 @@ class Scrollable extends StatefulWidget {
        assert(dragStartBehavior != null),
        assert(viewportBuilder != null),
        assert(excludeFromSemantics != null),
+       assert(semanticChildCount == null || semanticChildCount >= 0),
        super (key: key);
 
   /// The direction in which this widget scrolls.
@@ -643,6 +644,7 @@ class _ScrollSemantics extends SingleChildRenderObjectWidget {
     @required this.semanticChildCount,
     Widget child,
   }) : assert(position != null),
+       assert(semanticChildCount == null || semanticChildCount >= 0),
        super(key: key, child: child);
 
   final ScrollPosition position;

packages/flutter/test/widgets/scroll_view_test.dart

@@ -559,4 +559,33 @@ void main() {
     expect(tester.takeException(), isInstanceOf<Exception>());
     expect(finder, findsOneWidget);
   });
+
+  testWidgets('ListView.builder asserts on negative childCount', (WidgetTester tester) async {
+    expect(() => ListView.builder(
+      itemBuilder: (BuildContext context, int index) {
+        return const SizedBox();
+      },
+      itemCount: -1,
+    ), throwsA(isInstanceOf<AssertionError>()));
+  });
+
+  testWidgets('ListView.builder asserts on negative semanticChildCount', (WidgetTester tester) async {
+    expect(() => ListView.builder(
+      itemBuilder: (BuildContext context, int index) {
+        return const SizedBox();
+      },
+      itemCount: 1,
+      semanticChildCount: -1,
+    ), throwsA(isInstanceOf<AssertionError>()));
+  });
+
+  testWidgets('ListView.builder asserts on nonsensical childCount/semanticChildCount', (WidgetTester tester) async {
+    expect(() => ListView.builder(
+      itemBuilder: (BuildContext context, int index) {
+        return const SizedBox();
+      },
+      itemCount: 1,
+      semanticChildCount: 4,
+    ), throwsA(isInstanceOf<AssertionError>()));
+  });
 }