syncFields() should not sync something you set up in initState(). They are separate concepts.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1233143004 .

sdk/lib/widgets/popup_menu.dart

@@ -109,7 +109,6 @@ class PopupMenu extends AnimatedComponent {
     controller = source.controller;
     items = source.items;
     level = source.level;
-    _painter = source._painter;
     super.syncFields(source);
   }