move experimental/form_app to root of project (#623)

This allows us to reference this sample from other places.

See https://github.com/flutter/flutter/pull/70321 for more context.

form_app/lib/main.dart

@@ -0,0 +1,94 @@
+// 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 'package:http/http.dart' as http;
+
+import 'src/autofill.dart';
+import 'src/form_widgets.dart';
+import 'src/http/mock_client.dart';
+import 'src/sign_in_http.dart';
+import 'src/validation.dart';
+
+// Set up a mock HTTP client.
+final http.Client httpClient = MockClient();
+
+void main() {
+  runApp(FormApp());
+}
+
+final demos = [
+  Demo(
+    name: 'Sign in with HTTP',
+    route: '/signin_http',
+    builder: (context) => SignInHttpDemo(
+      httpClient: httpClient,
+    ),
+  ),
+  Demo(
+    name: 'Autofill',
+    route: '/autofill',
+    builder: (context) => AutofillDemo(),
+  ),
+  Demo(
+    name: 'Form widgets',
+    route: '/form_widgets',
+    builder: (context) => FormWidgetsDemo(),
+  ),
+  Demo(
+    name: 'Validation',
+    route: '/validation',
+    builder: (context) => FormValidationDemo(),
+  ),
+];
+
+class FormApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Form Samples',
+      theme: ThemeData(primarySwatch: Colors.teal),
+      routes: Map.fromEntries(demos.map((d) => MapEntry(d.route, d.builder))),
+      home: HomePage(),
+    );
+  }
+}
+
+class HomePage extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Form Samples'),
+      ),
+      body: ListView(
+        children: [...demos.map((d) => DemoTile(d))],
+      ),
+    );
+  }
+}
+
+class DemoTile extends StatelessWidget {
+  final Demo demo;
+
+  DemoTile(this.demo);
+
+  @override
+  Widget build(BuildContext context) {
+    return ListTile(
+      title: Text(demo.name),
+      onTap: () {
+        Navigator.pushNamed(context, demo.route);
+      },
+    );
+  }
+}
+
+class Demo {
+  final String name;
+  final String route;
+  final WidgetBuilder builder;
+
+  const Demo({this.name, this.route, this.builder});
+}