Don't recreate CoreTypes in transformers. Pass it in.

pkg/kernel/lib/transformations/insert_type_checks.dart

Index: pkg/kernel/lib/transformations/insert_type_checks.dart
diff --git a/pkg/kernel/lib/transformations/insert_type_checks.dart b/pkg/kernel/lib/transformations/insert_type_checks.dart
index 01db445a5e6a2056c54326b9046826a6a07fed93..a884442472a88daf750387fccec8dffbe269b771 100644
--- a/pkg/kernel/lib/transformations/insert_type_checks.dart
+++ b/pkg/kernel/lib/transformations/insert_type_checks.dart
@@ -17,13 +17,12 @@ import '../type_checker.dart';
 /// Ideally this should be done when initially generating kernel IR, but this
 /// is not practical at the moment.
 class InsertTypeChecks {
-  CoreTypes coreTypes;
+  final CoreTypes coreTypes;
   ClassHierarchy hierarchy;
 
-  InsertTypeChecks({this.coreTypes, this.hierarchy});
+  InsertTypeChecks(this.coreTypes, {this.hierarchy});
 
   void transformProgram(Program program) {
-    coreTypes ??= new CoreTypes(program);
     hierarchy ??= new ClassHierarchy(program);
     new CheckInsertingTypeChecker(coreTypes, hierarchy).checkProgram(program);
   }