add type parameter back to PageRouteFactory (#26559)

packages/flutter/lib/src/cupertino/app.dart

@@ -274,10 +274,8 @@ class _CupertinoAppState extends State<CupertinoApp> {
           key: GlobalObjectKey(this),
           navigatorKey: widget.navigatorKey,
           navigatorObservers: _navigatorObservers,
-          // TODO(dnfield): when https://github.com/dart-lang/sdk/issues/34572 is resolved
-          // this can use type arguments again
-          pageRouteBuilder: (RouteSettings settings, WidgetBuilder builder) =>
-            CupertinoPageRoute<dynamic>(settings: settings, builder: builder),
+          pageRouteBuilder: <T>(RouteSettings settings, WidgetBuilder builder) =>
+            CupertinoPageRoute<T>(settings: settings, builder: builder),
           home: widget.home,
           routes: widget.routes,
           initialRoute: widget.initialRoute,

packages/flutter/lib/src/material/app.dart

@@ -411,10 +411,8 @@ class _MaterialAppState extends State<MaterialApp> {
         key: GlobalObjectKey(this),
         navigatorKey: widget.navigatorKey,
         navigatorObservers: _navigatorObservers,
-        // TODO(dnfield): when https://github.com/dart-lang/sdk/issues/34572 is resolved
-        // this can use type arguments again
-        pageRouteBuilder: (RouteSettings settings, WidgetBuilder builder) =>
-          MaterialPageRoute<dynamic>(settings: settings, builder: builder),
+        pageRouteBuilder: <T>(RouteSettings settings, WidgetBuilder builder) =>
+          MaterialPageRoute<T>(settings: settings, builder: builder),
         home: widget.home,
         routes: widget.routes,
         initialRoute: widget.initialRoute,

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

@@ -83,9 +83,7 @@ typedef GenerateAppTitle = String Function(BuildContext context);
 /// The signature of [WidgetsApp.pageRouteBuilder].
 ///
 /// Creates a [PageRoute] using the given [RouteSettings] and [WidgetBuilder].
-// TODO(dnfield): when https://github.com/dart-lang/sdk/issues/34572 is resolved
-// this can use type arguments again
-typedef PageRouteFactory = PageRoute<dynamic> Function(RouteSettings settings, WidgetBuilder builder);
+typedef PageRouteFactory = PageRoute<T> Function<T>(RouteSettings settings, WidgetBuilder builder);
 
 /// A convenience class that wraps a number of widgets that are commonly
 /// required for an application.
@@ -754,7 +752,7 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
       assert(widget.pageRouteBuilder != null,
         'The default onGenerateRoute handler for WidgetsApp must have a '
         'pageRouteBuilder set if the home or routes properties are set.');
-      final Route<dynamic> route = widget.pageRouteBuilder(
+      final Route<dynamic> route = widget.pageRouteBuilder<dynamic>(
         settings,
         pageContentBuilder,
       );