Pass ClassHierarchy instead of creating it.

pkg/kernel/lib/transformations/insert_covariance_checks.dart

Index: pkg/kernel/lib/transformations/insert_covariance_checks.dart
diff --git a/pkg/kernel/lib/transformations/insert_covariance_checks.dart b/pkg/kernel/lib/transformations/insert_covariance_checks.dart
index 82f9682773a57ba63905776de387724c8c1a70fd..4367dd846fa3e8db5dabca82db2a4d2448c62894 100644
--- a/pkg/kernel/lib/transformations/insert_covariance_checks.dart
+++ b/pkg/kernel/lib/transformations/insert_covariance_checks.dart
@@ -70,10 +70,9 @@ class InsertCovarianceChecks {
   /// 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.