Give more control to SliverChildDelegate (#7885)

This patch moves the resonsibility for wrapping repaint boundaries
around children to SliverChildDelegate, which means delegates can choose
whether or no to use repaint boundaries.

Also introduce SliverChildBuilderDelegate to make it easier to use the
builder pattern with sliver lists.

This functionality will be used by date picker, which wants to use a
SliverGrid but doesn't need a repaint boundary around every day in a
month grid.

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

@@ -41,6 +41,31 @@ abstract class SliverChildDelegate {
   bool shouldRebuild(@checked SliverChildDelegate oldDelegate);
 }
 
+class SliverChildBuilderDelegate extends SliverChildDelegate {
+  const SliverChildBuilderDelegate(this.builder, { this.childCount });
+
+  final IndexedWidgetBuilder builder;
+
+  final int childCount;
+
+  @override
+  Widget build(BuildContext context, int index) {
+    assert(builder != null);
+    if (index < 0 || (childCount != null && index >= childCount))
+      return null;
+    final Widget child = builder(context, index);
+    if (child == null)
+      return null;
+    return new RepaintBoundary.wrap(child, index);
+  }
+
+  @override
+  int get estimatedChildCount => childCount;
+
+  @override
+  bool shouldRebuild(@checked SliverChildListDelegate oldDelegate) => true;
+}
+
 // ///
 // /// In general building all the widgets in advance is not efficient. It is
 // /// better to create a delegate that builds them on demand by subclassing
@@ -63,7 +88,9 @@ class SliverChildListDelegate extends SliverChildDelegate {
     assert(children != null);
     if (index < 0 || index >= children.length)
       return null;
-    return children[index];
+    final Widget child = children[index];
+    assert(child != null);
+    return new RepaintBoundary.wrap(child, index);
   }
 
   @override
@@ -253,12 +280,7 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
   }
 
   Widget _build(int index) {
-    return _childWidgets.putIfAbsent(index, () {
-      Widget child = widget.delegate.build(this, index);
-      if (child == null)
-        return null;
-      return new RepaintBoundary.wrap(child, index);
-    });
+    return _childWidgets.putIfAbsent(index, () => widget.delegate.build(this, index));
   }
 
   @override