Default to mock version in web_dashboard (#505)

* use named constructors, set default app to mock version

* rename grind tasks

experimental/web_dashboard/lib/main_firebase.dart

@@ -0,0 +1,11 @@
+// Copyright 2020, the Flutter 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.
+
+import 'package:flutter/material.dart';
+
+import 'src/app.dart';
+
+void main() {
+  runApp(DashboardApp.firebase());
+}