Enable fading animation during page transitions. (#21394)

The average frame time of page transitions on Moto G4 is now very
close to 16ms (the last 10 measurements on our dashboard are
between 15.5ms to 16.7ms and half of them are below 16ms).

It is now much faster than when we disabled it (which was at about
35ms). So I think that we should be able to enable it by default.
I'll leave the flag there until we implement the retained rendering
to bring the frame time comfortably below 16ms.

See https://github.com/flutter/flutter/issues/13736

bin/internal/goldens.version

-1a999092d10a22bc700214b257cd4890c5800078
+157b8cebd10c8f8308540ded5dadae502febd48d

examples/flutter_gallery/lib/main.dart

@@ -14,8 +14,5 @@ void main() {
   // Temporary debugging hook for https://github.com/flutter/flutter/issues/17888
   debugInstrumentationEnabled = true;
 
-  // Overriding https://github.com/flutter/flutter/issues/13736 for better
-  // visual effect at the cost of performance.
-  MaterialPageRoute.debugEnableFadingRoutes = true; // ignore: deprecated_member_use
   runApp(const GalleryApp());
 }

examples/flutter_gallery/test_driver/scroll_perf.dart

@@ -10,6 +10,5 @@ void main() {
   enableFlutterDriverExtension();
   // As in lib/main.dart: overriding https://github.com/flutter/flutter/issues/13736
   // for better visual effect at the cost of performance.
-  MaterialPageRoute.debugEnableFadingRoutes = true; // ignore: deprecated_member_use
   runApp(const GalleryApp(testMode: true));
 }

examples/flutter_gallery/test_driver/transitions_perf.dart

@@ -21,6 +21,5 @@ void main() {
   enableFlutterDriverExtension(handler: _handleMessages);
   // As in lib/main.dart: overriding https://github.com/flutter/flutter/issues/13736
   // for better visual effect at the cost of performance.
-  MaterialPageRoute.debugEnableFadingRoutes = true; // ignore: deprecated_member_use
   runApp(const GalleryApp(testMode: true));
 }

examples/flutter_gallery/test_memory/back_button.dart

@@ -28,7 +28,6 @@ class LifecycleObserver extends WidgetsBindingObserver {
 }
 
 Future<void> main() async {
-  MaterialPageRoute.debugEnableFadingRoutes = true; // ignore: deprecated_member_use
   runApp(const GalleryApp());
   await endOfAnimation();
   await new Future<Null>.delayed(const Duration(milliseconds: 50));

examples/flutter_gallery/test_memory/memory_nav.dart

@@ -23,7 +23,6 @@ Rect boundsFor(WidgetController controller, Finder item) {
 }
 
 Future<void> main() async {
-  MaterialPageRoute.debugEnableFadingRoutes = true; // ignore: deprecated_member_use
   final Completer<void> ready = new Completer<void>();
   runApp(new GestureDetector(
     onTap: () {

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

@@ -87,14 +87,6 @@ class MaterialPageRoute<T> extends PageRoute<T> {
     assert(opaque);
   }
 
-  /// Turns on the fading of routes during page transitions.
-  ///
-  /// This is currently disabled by default because of performance issues on
-  /// low-end phones. Eventually these issues will be resolved and this flag
-  /// will be removed.
-  @Deprecated('This flag will eventually be removed once the performance issues are resolved. See: https://github.com/flutter/flutter/issues/13736')
-  static bool debugEnableFadingRoutes = false;
-
   /// Builds the primary contents of the route.
   final WidgetBuilder builder;
 
@@ -175,7 +167,7 @@ class MaterialPageRoute<T> extends PageRoute<T> {
       return new _MountainViewPageTransition(
         routeAnimation: animation,
         child: child,
-        fade: debugEnableFadingRoutes, // ignore: deprecated_member_use
+        fade: true,
       );
     }
   }

packages/flutter/test/material/page_test.dart

@@ -40,7 +40,7 @@ void main() {
     // Animation begins 3/4 of the way up the page.
     expect(widget2TopLeft.dy < widget2Size.height / 4.0, true);
     // Animation starts with page 2 being near transparent.
-    expect(widget2Opacity.opacity.value < 0.01, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
+    expect(widget2Opacity.opacity.value < 0.01, true);
 
     await tester.pump(const Duration(milliseconds: 300));
 
@@ -59,7 +59,7 @@ void main() {
     // Page 2 starts to move down.
     expect(widget1TopLeft.dy < widget2TopLeft.dy, true);
     // Page 2 starts to lose opacity.
-    expect(widget2Opacity.opacity.value < 1.0, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
+    expect(widget2Opacity.opacity.value < 1.0, true);
 
     await tester.pump(const Duration(milliseconds: 300));