Update tests/language/vm/causal_async_exception_stack_test

tests/language/vm/causal_async_exception_stack_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:unittest/unittest.dart';
 
 thrower() async {
   throw 'oops';
 }
 
@@ skipping 52 matching lines @@
           ]));
   }
 
   // Test for correct linkage.
   try {
     await thrower();
   } catch(e, st) {
     expect(st.toString(), stringContainsInOrder([
            'thrower', '.dart:8',
            '<asynchronous suspension>',
-           'main', '.dart:67',
+           'main', '.dart:68',
            ]));
   }
 }