Ease Material route animations redux (#5509)

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

@@ -447,7 +447,7 @@ class HeroController extends NavigatorObserver {
       if (previousRoute is PageRoute<dynamic>) // could be null
         _from = previousRoute;
       _to = route;
-      _animation = new CurvedAnimation(parent: route.animation, curve: Curves.fastOutSlowIn);
+      _animation = route.animation;
       _checkForHeroQuest();
     }
   }
@@ -461,7 +461,7 @@ class HeroController extends NavigatorObserver {
       if (previousRoute is PageRoute<dynamic>) {
         _to = previousRoute;
         _from = route;
-        _animation = new CurvedAnimation(parent: route.animation, curve: Curves.fastOutSlowIn);
+        _animation = route.animation;
         _checkForHeroQuest();
       }
     }
@@ -516,19 +516,18 @@ class HeroController extends NavigatorObserver {
     Map<Object, HeroHandle> heroesTo = Hero.of(_to.subtreeContext);
     _to.offstage = false;
 
-    Animation<double> animation = _animation;
+    Animation<double> animation = _animation; // The route's animation.
     Curve curve = Curves.fastOutSlowIn;
     if (animation.status == AnimationStatus.reverse) {
       animation = new ReverseAnimation(animation);
       curve = new Interval(animation.value, 1.0, curve: curve);
     }
+    animation = new CurvedAnimation(parent: animation, curve: curve);
 
     _party.animate(heroesFrom, heroesTo, _getAnimationArea(navigator.context));
     _removeHeroesFromOverlay();
-    _party.setAnimation(new CurvedAnimation(
-      parent: animation,
-      curve: curve
-    ));
+    _party.setAnimation(animation);
+
     for (_HeroQuestState hero in _party._heroes) {
       hero.overlayEntry = _addHeroToOverlay(
         (BuildContext context) => hero.build(navigator.context, animation),

packages/flutter/test/widget/heroes_test.dart

@@ -177,6 +177,46 @@ void main() {
     await tester.pump(new Duration(seconds: 1));
   });
 
+  testWidgets('Heroes animation is fastOutSlowIn', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(routes: routes));
+    await tester.tap(find.text('two'));
+    await tester.pump(); // begin navigation
+
+    // Expect the height of the secondKey Hero to vary from 100 to 150
+    // over duration and according to curve.
+
+    final Duration duration = const Duration(milliseconds: 300);
+    final Curve curve = Curves.fastOutSlowIn;
+    final double initialHeight = tester.getSize(find.byKey(firstKey)).height;
+    final double finalHeight = tester.getSize(find.byKey(secondKey)).height;
+    final double deltaHeight = finalHeight - initialHeight;
+    final double epsilon = 0.001;
+
+    await tester.pump(duration * 0.25);
+    expect(
+      tester.getSize(find.byKey(secondKey)).height,
+      closeTo(curve.transform(0.25) * deltaHeight + initialHeight, epsilon)
+    );
+
+    await tester.pump(duration * 0.25);
+    expect(
+      tester.getSize(find.byKey(secondKey)).height,
+      closeTo(curve.transform(0.50) * deltaHeight + initialHeight, epsilon)
+    );
+
+    await tester.pump(duration * 0.25);
+    expect(
+      tester.getSize(find.byKey(secondKey)).height,
+      closeTo(curve.transform(0.75) * deltaHeight + initialHeight, epsilon)
+    );
+
+    await tester.pump(duration * 0.25);
+    expect(
+      tester.getSize(find.byKey(secondKey)).height,
+      closeTo(curve.transform(1.0) * deltaHeight + initialHeight, epsilon)
+    );
+  });
+
   testWidgets('Heroes are not interactive', (WidgetTester tester) async {
     List<String> log = <String>[];
 
@@ -246,5 +286,4 @@ void main() {
     await tester.tap(find.text('bar'));
     expect(log, equals(<String>['bar']));
   });
-
 }