Allow isolate/mandel_* tests to run for longer than the default 20 seconds.

dart/tests/isolate/mandel_isolate_stream_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);
   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 118 matching lines @@
 
   stream.listen((message) {
     if (isFirst) {
       isFirst = false;
       replyTo = message;
       return;
     }
     replyTo.add(processLine(message));
   });
 }