Limit number of browsers opened by test script to 15.

tools/testing/dart/test_configurations.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 test_configurations;
 
 import "dart:async";
 import 'dart:io';
 import "dart:math" as math;
 
@@ skipping 142 matching lines @@
       // browser.
       maxBrowserProcesses = 1;
     } else if (conf['runtime'].startsWith('safari')) {
       // Safari does not allow us to run from a fresh profile, so we can only
       // use one browser. Additionally, you can not start two simulators
       // for mobile safari simultainiously.
       maxBrowserProcesses = 1;
     } else if (conf['runtime'] == 'chrome' &&
                Platform.operatingSystem == 'macos') {
       // Chrome on mac results in random timeouts.
+      // Issue: https://github.com/dart-lang/sdk/issues/23891
+      // This change does not fix the problem.
       maxBrowserProcesses = math.max(1, maxBrowserProcesses ~/ 2);
+    } else if (conf['runtime'] != 'drt') {
+      // Even on machines with more than 16 processors, don't open more
+      // than 15 browser instances, to avoid overloading the machine.
+      // This is especially important when running locally on powerful
+      // desktops.
+      maxBrowserProcesses = math.min(maxBrowserProcesses, 15);
     }
 
     // If we specifically pass in a suite only run that.
     if (conf['suite_dir'] != null) {
       var suite_path = new Path(conf['suite_dir']);
       testSuites.add(new PKGTestSuite(conf, suite_path));
     } else {
       for (String key in selectors.keys) {
         if (key == 'co19') {
           testSuites.add(new Co19TestSuite(conf));
@@ skipping 108 matching lines @@
                      recordingOutputPath);
   }
 
   // Start all the HTTP servers required before starting the process queue.
   if (serverFutures.isEmpty) {
     startProcessQueue();
   } else {
     Future.wait(serverFutures).then((_) => startProcessQueue());
   }
 }