Start browsers sequentially in testing scripts, as needed.

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 2540 matching lines @@
 
     BrowserTest browserTest;
     if (browserCommand is BrowserHtmlTestCommand) {
       browserTest = new HtmlTest(browserCommand.url, callback, timeout,
           browserCommand.expectedMessages);
     } else {
       browserTest = new BrowserTest(browserCommand.url, callback, timeout);
     }
     _getBrowserTestRunner(browserCommand.browser, browserCommand.configuration)
         .then((testRunner) {
-      testRunner.queueTest(browserTest);
+      testRunner.enqueueTest(browserTest);
     });
 
     return completer.future;
   }
 
   Future<BrowserTestRunner> _getBrowserTestRunner(String browser,
-                                                  Map configuration) {
+                                                  Map configuration) async {
     var localIp = globalConfiguration['local_ip'];
-    var num_browsers = maxBrowserProcesses;
     if (_browserTestRunners[configuration] == null) {
       var testRunner = new BrowserTestRunner(
-            configuration, localIp, browser, num_browsers);
+            configuration, localIp, browser, maxBrowserProcesses);
       if (globalConfiguration['verbose']) {
         testRunner.logger = DebugLogger.info;
       }
       _browserTestRunners[configuration] = testRunner;
-      return testRunner.start().then((started) {
-        if (started) {
-          return testRunner;
-        }
-        print("Issue starting browser test runner");
-        io.exit(1);
-      });
+      await testRunner.start();
     }
-    return new Future.value(_browserTestRunners[configuration]);
+    return _browserTestRunners[configuration];
   }
 }
 
 class RecordingCommandExecutor implements CommandExecutor {
   TestCaseRecorder _recorder;
 
   RecordingCommandExecutor(Path path)
       : _recorder = new TestCaseRecorder(path);
 
   Future<CommandOutput> runCommand(node, ProcessCommand command, int timeout) {
@@ skipping 360 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }