Remove the ClassHierarchy.rootClass getter.

pkg/kernel/test/treeshaker_membench.dart

Index: pkg/kernel/test/treeshaker_membench.dart
diff --git a/pkg/kernel/test/treeshaker_membench.dart b/pkg/kernel/test/treeshaker_membench.dart
index 8ca99eaca9289afa7157d2fa8247436301d81cf3..596ef87b9165b5ab1ca4488ebf3af6f4f75b6770 100644
--- a/pkg/kernel/test/treeshaker_membench.dart
+++ b/pkg/kernel/test/treeshaker_membench.dart
@@ -59,7 +59,7 @@ main(List<String> args) {
   if (args.contains('-v')) {
     // Use of the list for something to avoid premature GC.
     for (var treeShaker in keepAlive) {
-      treeShaker.getClassRetention(hierarchy.rootClass);
+      treeShaker.getClassRetention(coreTypes.objectClass);
     }
   }
 }