Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

tests/compiler/dart2js/serialization/model_test_helper.dart

Index: tests/compiler/dart2js/serialization/model_test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/model_test_helper.dart b/tests/compiler/dart2js/serialization/model_test_helper.dart
index 57cfb2430502708451f3cfd713adc5596a4a0768..00fe7490ade517ab5f75ef86235895f4c03125b5 100644
--- a/tests/compiler/dart2js/serialization/model_test_helper.dart
+++ b/tests/compiler/dart2js/serialization/model_test_helper.dart
@@ -451,7 +451,9 @@ bool areInstantiationInfosEquivalent(
 
 bool areInstancesEquivalent(Instance instance1, Instance instance2,
     bool typeEquivalence(ResolutionDartType a, ResolutionDartType b)) {
-  return typeEquivalence(instance1.type, instance2.type) &&
+  ResolutionInterfaceType type1 = instance1.type;
+  ResolutionInterfaceType type2 = instance2.type;
+  return typeEquivalence(type1, type2) &&
       instance1.kind == instance2.kind &&
       instance1.isRedirection == instance2.isRedirection;
 }