Fix analyzer warning from test for issue21079.

tests/language/issue21079_test.dart

 // Copyright (c) 2013, 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.
 
 // Regression test case for http://dartbug.com/21079
 import 'dart:mirrors';
 import 'dart:isolate';
 import "package:expect/expect.dart";
 
 void main() {
   Expect.isTrue(reflectClass(MyException).superclass.reflectedType == IsolateSpawnException);
 
   Expect.isTrue(reflectClass(IsolateSpawnException).reflectedType == IsolateSpawnException);
 }
 
-class MyException extends IsolateSpawnException {}
+class MyException extends IsolateSpawnException {
+  MyException() : super("Test") {}
+}