Remove the ClassHierarchy.rootClass getter.

pkg/kernel/test/class_hierarchy_test.dart

Index: pkg/kernel/test/class_hierarchy_test.dart
diff --git a/pkg/kernel/test/class_hierarchy_test.dart b/pkg/kernel/test/class_hierarchy_test.dart
index b2d62c038533edd035453b9bcad4c58857d2ff17..ba65e724d569317a8a488c8528c86e25137a5d22 100644
--- a/pkg/kernel/test/class_hierarchy_test.dart
+++ b/pkg/kernel/test/class_hierarchy_test.dart
@@ -654,11 +654,6 @@ class B<T> extends self::A<self::B::T, core::bool> {}
         new InterfaceType(objectClass));
   }
 
-  void test_rootClass() {
-    addClass(new Class(name: 'A', supertype: objectSuper));
-    expect(hierarchy.rootClass, objectClass);
-  }
-
   void _assertOverridePairs(Class class_, List<String> expected) {
     List<String> overrideDescriptions = [];
     hierarchy.forEachOverridePair(class_,