Disable default browser check on firefox.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
===================================================================
--- tools/testing/dart/browser_controller.dart	(revision 23059)
+++ tools/testing/dart/browser_controller.dart	(working copy)
@@ -284,12 +284,15 @@
   const String binary = "firefox";
 
   const String enablePopUp =
-      "user_pref(\"dom.disable_open_during_load\", false);";
+      'user_pref("dom.disable_open_during_load", false);';
+  const String disableDefaultCheck =
+      'user_pref("browser.shell.checkDefaultBrowser", false);';
 
   Future _createPreferenceFile(var path) {
     var file = new File("${path.toString()}/user.js");
     var randomFile = file.openSync(FileMode.WRITE);
     randomFile.writeStringSync(enablePopUp);
+    randomFile.writeStringSync(disableDefaultCheck);
     randomFile.close();
   }