Add factory constructor to browser controller.

tools/testing/dart/test_runner.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.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 1631 matching lines @@
 
     for (var runner in runners) {
       if (!runner.active) return runner;
     }
     throw new Exception('Unable to find inactive batch runner.');
   }
 
   Future<BrowserTestRunner> _getBrowserTestRunner(TestCase test) {
     var local_ip = test.configuration['local_ip'];
     var runtime = test.configuration['runtime'];
-    var num_browsers = 1;//test.configuration['tasks'];
+    var num_browsers = test.configuration['tasks'];
     if (_browserTestRunners[runtime] == null) {
       var testRunner =
         new BrowserTestRunner(local_ip, runtime, num_browsers);
       testRunner.logger = DebugLogger.info;
       _browserTestRunners[runtime] = testRunner;
       return testRunner.start().then((started) {
         if (started) {
           return testRunner;
         }
         print("Issue starting browser test runner");
@@ skipping 247 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
   }
 }