Use entities in BackendClasses

tests/compiler/dart2js/simple_inferrer_final_field2_test.dart

Index: tests/compiler/dart2js/simple_inferrer_final_field2_test.dart
diff --git a/tests/compiler/dart2js/simple_inferrer_final_field2_test.dart b/tests/compiler/dart2js/simple_inferrer_final_field2_test.dart
index 2a3146203e93146288a2bdb613ecb3ecedb26d6b..08643af9bd7e2f9d9e973cd000f050315d90715b 100644
--- a/tests/compiler/dart2js/simple_inferrer_final_field2_test.dart
+++ b/tests/compiler/dart2js/simple_inferrer_final_field2_test.dart
@@ -37,9 +37,9 @@ void main() {
               typesInferrer.getTypeOfElement(element).containsOnly(type));
         }
 
-        checkFieldTypeInClass('A', 'intField',
-            compiler.backend.backendClasses.uint31Implementation);
-        checkFieldTypeInClass('A', 'stringField',
-            compiler.backend.backendClasses.stringImplementation);
+        checkFieldTypeInClass(
+            'A', 'intField', compiler.backend.backendClasses.uint31Class);
+        checkFieldTypeInClass(
+            'A', 'stringField', compiler.backend.backendClasses.stringClass);
       }));
 }