Replace run_selenium.py with Dart script that uses the browser controller for performance tests.

tools/testing/dart/perf_test_controller.js

Index: tools/testing/dart/perf_test_controller.js
diff --git a/tests/language/deferred_constraints_lib.dart b/tools/testing/dart/perf_test_controller.js
similarity index 55%
copy from tests/language/deferred_constraints_lib.dart
copy to tools/testing/dart/perf_test_controller.js
index cdc9d4d8fbddf56553e4e04f5d44eeac2534fa3a..f6990bb65c70cdfd1c31f40dc3422ec04c6ee151 100644
--- a/tests/language/deferred_constraints_lib.dart
+++ b/tools/testing/dart/perf_test_controller.js
@@ -2,10 +2,8 @@
 // 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.
 
-class C {}
+/// Helper file to make performance tests run with the Dart browser controller
+/// in test.dart.
 
-class Const {
-  const Const();
-}
-
-const constantInstance = const Const();
+// First import test_controller.js
+dartPrint('unittest-suite-wait-for-done');