Fix warning in mandel_isolate_test.

tests/isolate/mandel_isolate_test.dart

 // Copyright (c) 2012, 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.
 
 library MandelIsolateTest;
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:math';
 import '../../pkg/unittest/lib/unittest.dart';
 
 const TERMINATION_MESSAGE = -1;
 const N = 100;
 const ISOLATES = 20;
 
 main() {
   // Test is really slow in debug builds of the VM.
-  unittestConfiguration.timeout = const Duration(seconds: 480);
+  SimpleConfiguration configuration = unittestConfiguration;
+  configuration.timeout = const Duration(seconds: 480);
   test("Render Mandelbrot in parallel", () {
     final state = new MandelbrotState();
     state._validated.future.then(expectAsync1((result) {
       expect(result, isTrue);
     }));
     for (int i = 0; i < min(ISOLATES, N); i++) state.startClient(i);
   });
 }
 
 
@@ skipping 121 matching lines @@
     if (message != TERMINATION_MESSAGE) {
       int line = message[0];
       SendPort replyTo = message[1];
       replyTo.send(processLine(line));
     } else {
       port.close();
     }
   });
   replyPort.send(port.sendPort);
 }