Depend on stable flutter instead of master

lib/mail_view_router.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
+import 'package:reply/transition_page_builder.dart';
 
 import 'home.dart';
 import 'inbox.dart';
@@ -26,11 +27,12 @@ class MailViewRouterDelegate extends RouterDelegate<void>
           onPopPage: _handlePopPage,
           pages: [
             // TODO: Add Fade through transition between mailbox pages (Motion)
-            TransitionBuilderPage(
-              pageBuilder: (context, animation, secondaryAnimation) {
-                return InboxPage(destination: currentlySelectedInbox);
-              },
-            ),
+            CustomTransitionPageBuilder(
+              transitionKey: ValueKey(currentlySelectedInbox),
+              screen: InboxPage(
+                destination: currentlySelectedInbox,
+              ),
+            )
           ],
         );
       },

lib/router.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:reply/home.dart';
 import 'package:reply/search_page.dart';
+import 'package:reply/transition_page_builder.dart';
 
 import 'model/router_provider.dart';
 
@@ -47,16 +48,14 @@ class ReplyRouterDelegate extends RouterDelegate<ReplyRoutePath>
             onPopPage: _handlePopPage,
             pages: [
               // TODO: Add Shared Z-Axis transition from search icon to search view page (Motion)
-              TransitionBuilderPage(
-                pageBuilder: (context, animation, secondaryAnimation) {
-                  return const HomePage();
-                },
+              const CustomTransitionPageBuilder(
+                transitionKey: ValueKey('Home'),
+                screen: HomePage(),
               ),
               if (routePath is ReplySearchPath)
-                TransitionBuilderPage(
-                  pageBuilder: (context, animation, secondaryAnimation) {
-                    return const SearchPage();
-                  },
+                const CustomTransitionPageBuilder(
+                  transitionKey: ValueKey('Search'),
+                  screen: SearchPage(),
                 ),
             ],
           );

lib/transition_page_builder.dart

@@ -0,0 +1,22 @@
+import 'package:flutter/widgets.dart';
+
+// Prefer to use TransitionBuilderPage once it lands in stable.
+class CustomTransitionPageBuilder extends Page {
+  final Widget screen;
+  final ValueKey transitionKey;
+
+  const CustomTransitionPageBuilder(
+      {@required this.screen, @required this.transitionKey})
+      : assert(screen != null),
+        assert(transitionKey != null),
+        super(key: transitionKey);
+
+  @override
+  Route createRoute(BuildContext context) {
+    return PageRouteBuilder(
+        settings: this,
+        pageBuilder: (context, animation, secondaryAnimation) {
+          return screen;
+        });
+  }
+}