Extract ClassHierarchyImpl implementation from ClassHierarchy.

pkg/kernel/test/class_hierarchy_bench.dart

Index: pkg/kernel/test/class_hierarchy_bench.dart
diff --git a/pkg/kernel/test/class_hierarchy_bench.dart b/pkg/kernel/test/class_hierarchy_bench.dart
index 5f2f34f9ff18d2de545d2a0ab6e3e68ab3ee86ba..02d6b0cc93150004aaa917a1441ea1f0f5dfc434 100644
--- a/pkg/kernel/test/class_hierarchy_bench.dart
+++ b/pkg/kernel/test/class_hierarchy_bench.dart
@@ -55,13 +55,13 @@ main(List<String> args) {
   int hotBuildTime = watch.elapsedMilliseconds ~/ numBuildTrials;
 
   int hierarchyCount = int.parse(options['cycle']);
-  var hierarchies = <ClassHierarchy>[];
+  var hierarchies = <ClosedWorldClassHierarchy>[];
   for (int i = 0; i < hierarchyCount; i++) {
     hierarchies.add(buildHierarchy());
   }
 
   int currentHierarchy = 0;
-  ClassHierarchy getClassHierarchy() {
+  ClosedWorldClassHierarchy getClassHierarchy() {
     currentHierarchy = (currentHierarchy + 1) % hierarchies.length;
     return hierarchies[currentHierarchy];
   }