Ensure frame is scheduled when root widget is attached (#40105)

packages/flutter/lib/src/widgets/binding.dart

@@ -940,6 +940,9 @@ class RenderObjectToWidgetAdapter<T extends RenderObject> extends RenderObjectWi
       owner.buildScope(element, () {
         element.mount(null, null);
       });
+      // This is most likely the first time the framework is ready to produce
+      // a frame. Ensure that we are asked for one.
+      SchedulerBinding.instance.ensureVisualUpdate();
     } else {
       element._newWidget = this;
       element.markNeedsBuild();

packages/flutter/test/widgets/binding_attach_root_widget_test.dart

+// Copyright 2019 The Flutter Authors. 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/scheduler.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  test('attachRootWidget will schedule a frame', () async {
+    final WidgetsFlutterBinding binding = WidgetsFlutterBinding.ensureInitialized();
+    expect(SchedulerBinding.instance.hasScheduledFrame, isFalse);
+
+    binding.attachRootWidget(const Placeholder());
+    expect(SchedulerBinding.instance.hasScheduledFrame, isTrue);
+  });
+}