Remove ClassHierarchy factory constructor.

pkg/kernel/test/treeshaker_bench.dart

Index: pkg/kernel/test/treeshaker_bench.dart
diff --git a/pkg/kernel/test/treeshaker_bench.dart b/pkg/kernel/test/treeshaker_bench.dart
index 07f660ef9597d8f315be0dd577d3af4304c61ba2..9f9354c49b82721e3b3c2332b69c7e98e27970e2 100644
--- a/pkg/kernel/test/treeshaker_bench.dart
+++ b/pkg/kernel/test/treeshaker_bench.dart
@@ -50,7 +50,7 @@ void main(List<String> args) {
   ClassHierarchy buildClassHierarchy() {
     return options['basic']
         ? new BasicClassHierarchy(program)
-        : new ClassHierarchy(program);
+        : new ClosedWorldClassHierarchy(program);
   }
 
   CoreTypes coreTypes = new CoreTypes(program);