Make Shrine scroll fast (#4408)

This fix isn't completely statisfying because it has a scaling limit. The ideal
fix would actually viewport the tiles in the grid. However, this fix is much
easier at the moment.

Fixes #4395

examples/flutter_gallery/lib/demo/shrine/shrine_home.dart

@@ -268,6 +268,7 @@ class _ShrineHomeState extends State<ShrineHome> {
     final Product featured = _products.firstWhere((Product product) => product.featureDescription != null);
     return new ShrinePage(
       body: new ScrollableViewport(
+        child: new RepaintBoundary(
           child: new Column(
             children: <Widget>[
               new Container(
@@ -281,15 +282,18 @@ class _ShrineHomeState extends State<ShrineHome> {
                 padding: const EdgeInsets.all(8.0),
                 tileAspectRatio: 160.0 / 216.0, // width/height
                 children: _products.map((Product product) {
-                return new ProductItem(
+                  return new RepaintBoundary(
+                    child: new ProductItem(
                       product: product,
                       onPressed: () { showOrderPage(product); }
+                    )
                   );
                 }).toList()
               )
             ]
           )
         )
+      )
     );
   }
 }

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

@@ -7,6 +7,7 @@ import 'dart:ui' show Offset;
 
 import 'package:flutter/painting.dart';
 import 'package:vector_math/vector_math_64.dart';
+import 'package:meta/meta.dart';
 import 'package:mojo_services/mojo/gfx/composition/scene_token.mojom.dart' as mojom;
 
 import 'debug.dart';
@@ -30,6 +31,7 @@ abstract class Layer {
   Layer _previousSibling;
 
   /// Removes this layer from its parent layer's child list
+  @mustCallSuper
   void detach() {
     if (_parent != null)
       _parent._remove(this);
@@ -94,12 +96,15 @@ abstract class Layer {
   }
 
   /// Add additional information to the given description for use by [toStringDeep].
+  @protected
+  @mustCallSuper
   void debugFillDescription(List<String> description) {
     if (debugCreator != null)
       description.add('creator: $debugCreator');
   }
 
   /// Returns a description of this layer's children for use by [toStringDeep].
+  @protected
   String debugDescribeChildren(String prefix) => '';
 }