Ease Material route animations (#5487)

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

@@ -18,8 +18,11 @@ class _MaterialPageTransition extends AnimatedWidget {
     this.child
   }) : super(
     key: key,
-    animation: _kMaterialPageTransitionTween.animate(new CurvedAnimation(parent: animation, curve: Curves.fastOutSlowIn))
-  );
+    animation: _kMaterialPageTransitionTween.animate(new CurvedAnimation(
+      parent: animation, // The route's linear 0.0 - 1.0 animation.
+      curve: Curves.fastOutSlowIn
+    )
+  ));
 
   final Widget child;
 

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 = route.animation;
+      _animation = new CurvedAnimation(parent: route.animation, curve: Curves.fastOutSlowIn);
       _checkForHeroQuest();
     }
   }
@@ -461,7 +461,7 @@ class HeroController extends NavigatorObserver {
       if (previousRoute is PageRoute<dynamic>) {
         _to = previousRoute;
         _from = route;
-        _animation = route.animation;
+        _animation = new CurvedAnimation(parent: route.animation, curve: Curves.fastOutSlowIn);
         _checkForHeroQuest();
       }
     }