Removed unnecessary Shrine hero logic (#5394)

* Removed unnecessary Shrine hero logic

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

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:collection' show HashSet;
 
 import 'package:flutter/material.dart';
 
@@ -265,8 +264,7 @@ class ProductItem extends StatelessWidget {
                 height: 144.0,
                 padding: const EdgeInsets.symmetric(horizontal: 8.0),
                 child: new Hero(
-                    tag: productHeroTag,
-                    key: new ObjectKey(product),
+                    tag: product.tag,
                     child: new Image.asset(product.imageAsset, fit: ImageFit.contain)
                   )
                 ),
@@ -306,12 +304,8 @@ class _ShrineHomeState extends State<ShrineHome> {
   void showOrderPage(Product product) {
     final Order order = _shoppingCart[product] ?? new Order(product: product);
     final Completer<Order> completer = new Completer<Order>();
-    final Key productKey = new ObjectKey(product);
-    final Set<Key> mostValuableKeys = new HashSet<Key>();
-    mostValuableKeys.add(productKey);
     Navigator.push(context, new ShrineOrderRoute(
       order: order,
-      settings: new RouteSettings(mostValuableKeys: mostValuableKeys),
       completer: completer,
       builder: (BuildContext context) {
         return new OrderPage(

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

@@ -40,7 +40,7 @@ class OrderItem extends StatelessWidget {
                 width: 248.0,
                 height: 248.0,
                 child: new Hero(
-                  tag: productHeroTag,
+                  tag: product.tag,
                   child: new Image.asset(product.imageAsset, fit: ImageFit.contain)
                 )
               )

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

@@ -4,8 +4,6 @@
 
 import 'dart:ui' show hashValues;
 
-const String productHeroTag = 'Product';
-
 class Vendor {
   const Vendor({
     this.name,
@@ -48,6 +46,7 @@ class Product {
   final double price;
   final Vendor vendor;
 
+  String get tag => name; // Unique value for Heroes
   String get priceString => '\$${price.floor()}';
 
   bool isValid() {

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

@@ -387,7 +387,7 @@ class HeroParty {
       assert(to == null || to.sourceStates.length == 1);
       assert(to == null || to.currentTurns.floor() == to.currentTurns);
       HeroState targetState = to != null ? to.sourceStates.elementAt(0) : null;
-      Set<HeroState> sourceStates = from != null ? from.sourceStates : new HashSet<HeroState>();
+      Set<HeroState> sourceStates = from?.sourceStates ?? new HashSet<HeroState>();
       sourceStates.remove(targetState);
       Rect sourceRect = from?.currentRect ?? to.currentRect.center & Size.zero;
       Rect targetRect = to?.currentRect ?? from.currentRect.center & Size.zero;
@@ -545,7 +545,7 @@ class HeroController extends NavigatorObserver {
 
   void _updateQuest(Duration timeStamp) {
     if (navigator == null) {
-      // The navigator has been removed for this end-of-frame callback was called.
+      // The navigator was removed before this end-of-frame callback was called.
       return;
     }
     Set<Key> mostValuableKeys = _getMostValuableKeys();