Wrap route transitions above the transition as well to prevent repaints (#15534)

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

@@ -466,6 +466,9 @@ class _ModalScopeState extends State<_ModalScope> {
         offstage: widget.route.offstage,
         child: new IgnorePointer(
           ignoring: widget.route.animation?.status == AnimationStatus.reverse,
+          // Keep the transition between repaint boundaries so we don't trigger
+          // deep repaints of the render tree above or below the transition.
+          child: new RepaintBoundary(
             child: widget.route.buildTransitions(
               context,
               widget.route.animation,
@@ -485,6 +488,7 @@ class _ModalScopeState extends State<_ModalScope> {
             ),
           ),
         ),
+      ),
     );
   }
 }