Switch complex_layout back to LazyBlock (#7940)

We need to study these benchmark numbers more carefully. See #7937

dev/benchmarks/complex_layout/lib/main.dart

@@ -52,6 +52,24 @@ class ComplexLayout extends StatefulWidget {
   static ComplexLayoutState of(BuildContext context) => context.ancestorStateOfType(const TypeMatcher<ComplexLayoutState>());
 }
 
+class FancyItemDelegate extends LazyBlockDelegate {
+  @override
+  Widget buildItem(BuildContext context, int index) {
+    if (index % 2 == 0)
+      return new FancyImageItem(index, key: new ValueKey<int>(index));
+    else
+      return new FancyGalleryItem(index, key: new ValueKey<int>(index));
+  }
+
+  @override
+  bool shouldRebuild(FancyItemDelegate oldDelegate) => false;
+
+  @override
+  double estimateTotalExtent(int firstIndex, int lastIndex, double minOffset, double firstStartOffset, double lastEndOffset) {
+    return double.INFINITY;
+  }
+}
+
 class ComplexLayoutState extends State<ComplexLayout> {
   @override
   Widget build(BuildContext context) {
@@ -73,15 +91,10 @@ class ComplexLayoutState extends State<ComplexLayout> {
       body: new Column(
         children: <Widget>[
           new Expanded(
-            child: new ListView.builder(
+            child: new LazyBlock(
               key: new Key('main-scroll'), // this key is used by the driver test
-              itemBuilder: (BuildContext context, int index) {
-                if (index % 2 == 0)
-                  return new FancyImageItem(index, key: new ValueKey<int>(index));
-                else
-                  return new FancyGalleryItem(index, key: new ValueKey<int>(index));
-              },
-            ),
+              delegate: new FancyItemDelegate(),
+            )
           ),
           new BottomBar(),
         ],