Fix expectation in test accessing private field.

Author: natebiggs

dwds/test/evaluate_common.dart

@@ -362,9 +362,10 @@ void testAll({
             'instance._field',
           );
 
+          expect(result, matchInstanceRefKind('String'));
           expect(
             result,
-            matchErrorRef(contains("The getter '_field' isn't defined")),
+            matchInstanceRef(contains("NoSuchMethodError: '_field")),
           );
         });
       });