Limit number of browsers opened by test script to 15.

tools/testing/dart/test_configurations.dart

Index: tools/testing/dart/test_configurations.dart
diff --git a/tools/testing/dart/test_configurations.dart b/tools/testing/dart/test_configurations.dart
index f4ea6d636857d4555445ddaeca9183e474bcf906..52f5372a139ec1a963e4cced455f11d3e86a4669 100644
--- a/tools/testing/dart/test_configurations.dart
+++ b/tools/testing/dart/test_configurations.dart
@@ -160,7 +160,15 @@ void testConfigurations(List<Map> configurations) {
     } 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.