Remove the ClassHierarchy.rootClass getter.

pkg/kernel/lib/class_hierarchy.dart

Index: pkg/kernel/lib/class_hierarchy.dart
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index 08283d8029ae26e62c5aff99a64c4f9826f14c03..4c79bacab42d57ced1c29b620b729c33f195023b 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -21,8 +21,6 @@ abstract class ClassHierarchy {
   /// The iterable is ordered so that classes occur after their super classes.
   Iterable<Class> get classes;
 
-  Class get rootClass;
-
   /// Returns the index of [class_] in the [classes] list.
   int getClassIndex(Class class_);
 
@@ -136,8 +134,6 @@ class ClosedWorldClassHierarchy implements ClassHierarchy {
   ClosedWorldClassHierarchy(Program program)
       : this._internal(program, _countClasses(program));
 
-  Class get rootClass => classes[0];
-
   @override
   int getClassIndex(Class class_) => _infoFor[class_].topologicalIndex;
 
@@ -472,6 +468,7 @@ class ClosedWorldClassHierarchy implements ClassHierarchy {
 
     // Run a downward traversal from the root, compute preorder numbers for
     // each class, and build their subtype sets as interval lists.
+    var rootClass = classes[0];
     _topDownSortVisit(_infoFor[rootClass]);
 
     for (int i = 0; i < classes.length; ++i) {