Pass ClassHierarchy instead of creating it.

pkg/kernel/lib/transformations/insert_covariance_checks.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.transformations.insert_covariance_checks;
 
 import '../class_hierarchy.dart';
 import '../clone.dart';
 import '../core_types.dart';
 import '../kernel.dart';
 import '../log.dart';
@@ skipping 52 matching lines @@
   final Map<Member, Procedure> unsafeMemberEntryPoint = <Member, Procedure>{};
 
   /// Members that may be invoked through a checked entry point.
   ///
   /// Note that these members are not necessarily unsafe, because a safe member
   /// can override an unsafe member, and thereby be invoked through a checked
   /// entry point.  This set is not therefore not the same as the set of keys
   /// in [unsafeMemberEntryPoint].
   final Set<Member> membersWithCheckedEntryPoint = new Set<Member>();
 
-  InsertCovarianceChecks(this.coreTypes, {this.hierarchy});
+  InsertCovarianceChecks(this.coreTypes, this.hierarchy);
 
   void transformProgram(Program program) {
-    hierarchy ??= new ClassHierarchy(program);
     types = new TypeEnvironment(coreTypes, hierarchy);
     // We transform every class before their subtypes.
     // This ensures that transitive overrides are taken into account.
     hierarchy.classes.forEach(transformClass);
 
     program.accept(new _CallTransformer(this));
   }
 
   void transformClass(Class class_) {
     new _ClassTransformer(class_, this).transformClass();
@@ skipping 428 matching lines @@
   @override
   visitPropertySet(PropertySet node) {
     var target = getChecked(node.receiver, node.interfaceTarget);
     if (target != null) {
       node.interfaceTarget = target;
       node.name = target.name;
     }
     node.visitChildren(this);
   }
 }