Delay Firefox browser startup in test scripts, to flatten load.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 27765697ea3b74db45fe86f46381cd51199c2df6..58cefc4b44c5c5917016c9338c21ac564142bbfc 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -767,7 +767,10 @@ class Firefox extends Browser {
         ];
         var environment = new Map<String, String>.from(Platform.environment);
         environment["MOZ_CRASHREPORTER_DISABLE"] = "1";
-        return startBrowserProcess(_binary, args, environment: environment);
+        // TODO(26060): Remove delay when dart2js stops hanging under load.
+        return new Future.delayed(new Duration(seconds: 3)).then((_) {

[ahe, 2016/04/25 11:17:21]

return new Future.delayed(new Duration(seconds: 3), () {
  return startBrowserProcess(_binary, args, environment: environment);
});

+          return startBrowserProcess(_binary, args, environment: environment);
+        });
       });
     }).catchError((e) {
       _logEvent("Running $_binary --version failed with $e");