Cleanup widgets/sliver_persistent_header.dart with resolution of dart-lang/sdk#31543 (#35828)

* Remove extra classes.

Now that https://github.com/dart-lang/sdk/issues/31543 is fixed, we
can remove extra classes.

* Add snapConfiguration to parameters for _RenderSliverFloatingPersistentHeaderForWidgets and _RenderSliverFloatingPinnedPersistentHeaderForWidgets constructors.

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

@@ -284,10 +284,7 @@ class _SliverScrollingPersistentHeader extends _SliverPersistentHeaderRenderObje
   }
 }
 
-// This class exists to work around https://github.com/dart-lang/sdk/issues/31543
-abstract class _RenderSliverScrollingPersistentHeader extends RenderSliverScrollingPersistentHeader { }
-
-class _RenderSliverScrollingPersistentHeaderForWidgets extends _RenderSliverScrollingPersistentHeader
+class _RenderSliverScrollingPersistentHeaderForWidgets extends RenderSliverScrollingPersistentHeader
   with _RenderSliverPersistentHeaderForWidgetsMixin { }
 
 class _SliverPinnedPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
@@ -302,10 +299,7 @@ class _SliverPinnedPersistentHeader extends _SliverPersistentHeaderRenderObjectW
   }
 }
 
-// This class exists to work around https://github.com/dart-lang/sdk/issues/31543
-abstract class _RenderSliverPinnedPersistentHeader extends RenderSliverPinnedPersistentHeader { }
-
-class _RenderSliverPinnedPersistentHeaderForWidgets extends _RenderSliverPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }
+class _RenderSliverPinnedPersistentHeaderForWidgets extends RenderSliverPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }
 
 class _SliverFloatingPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
   const _SliverFloatingPersistentHeader({
@@ -315,10 +309,7 @@ class _SliverFloatingPersistentHeader extends _SliverPersistentHeaderRenderObjec
 
   @override
   _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
-    // Not passing this snapConfiguration as a constructor parameter to avoid the
-    // additional layers added due to https://github.com/dart-lang/sdk/issues/31543
-    return _RenderSliverFloatingPersistentHeaderForWidgets()
-      ..snapConfiguration = delegate.snapConfiguration;
+    return _RenderSliverFloatingPersistentHeaderForWidgets(snapConfiguration: delegate.snapConfiguration);
   }
 
   @override
@@ -327,10 +318,12 @@ class _SliverFloatingPersistentHeader extends _SliverPersistentHeaderRenderObjec
   }
 }
 
-// This class exists to work around https://github.com/dart-lang/sdk/issues/31543
-abstract class _RenderSliverFloatingPinnedPersistentHeader extends RenderSliverFloatingPinnedPersistentHeader { }
-
-class _RenderSliverFloatingPinnedPersistentHeaderForWidgets extends _RenderSliverFloatingPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }
+class _RenderSliverFloatingPinnedPersistentHeaderForWidgets extends RenderSliverFloatingPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin {
+  _RenderSliverFloatingPinnedPersistentHeaderForWidgets({
+    RenderBox child,
+    FloatingHeaderSnapConfiguration snapConfiguration,
+  }) : super(child: child, snapConfiguration: snapConfiguration);
+}
 
 class _SliverFloatingPinnedPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
   const _SliverFloatingPinnedPersistentHeader({
@@ -340,10 +333,7 @@ class _SliverFloatingPinnedPersistentHeader extends _SliverPersistentHeaderRende
 
   @override
   _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
-    // Not passing this snapConfiguration as a constructor parameter to avoid the
-    // additional layers added due to https://github.com/dart-lang/sdk/issues/31543
-    return _RenderSliverFloatingPinnedPersistentHeaderForWidgets()
-      ..snapConfiguration = delegate.snapConfiguration;
+    return _RenderSliverFloatingPinnedPersistentHeaderForWidgets(snapConfiguration: delegate.snapConfiguration);
   }
 
   @override
@@ -352,7 +342,9 @@ class _SliverFloatingPinnedPersistentHeader extends _SliverPersistentHeaderRende
   }
 }
 
-// This class exists to work around https://github.com/dart-lang/sdk/issues/31543
-abstract class _RenderSliverFloatingPersistentHeader extends RenderSliverFloatingPersistentHeader { }
-
-class _RenderSliverFloatingPersistentHeaderForWidgets extends _RenderSliverFloatingPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }
+class _RenderSliverFloatingPersistentHeaderForWidgets extends RenderSliverFloatingPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin {
+  _RenderSliverFloatingPersistentHeaderForWidgets({
+    RenderBox child,
+    FloatingHeaderSnapConfiguration snapConfiguration,
+  }) : super(child: child, snapConfiguration: snapConfiguration);
+}