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/src/sign_in_http.dart

@@ -0,0 +1,124 @@
+// 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 'dart:convert';
+
+import 'package:http/http.dart' as http;
+import 'package:flutter/material.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+part 'sign_in_http.g.dart';
+
+@JsonSerializable()
+class FormData {
+  String email;
+  String password;
+
+  FormData({
+    this.email,
+    this.password,
+  });
+
+  factory FormData.fromJson(Map<String, dynamic> json) =>
+      _$FormDataFromJson(json);
+
+  Map<String, dynamic> toJson() => _$FormDataToJson(this);
+}
+
+class SignInHttpDemo extends StatefulWidget {
+  final http.Client httpClient;
+
+  SignInHttpDemo({
+    this.httpClient,
+  });
+
+  @override
+  _SignInHttpDemoState createState() => _SignInHttpDemoState();
+}
+
+class _SignInHttpDemoState extends State<SignInHttpDemo> {
+  FormData formData = FormData();
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Sign in Form'),
+      ),
+      body: Form(
+        child: Scrollbar(
+          child: SingleChildScrollView(
+            padding: EdgeInsets.all(16),
+            child: Column(
+              children: [
+                ...[
+                  TextFormField(
+                    decoration: InputDecoration(
+                      filled: true,
+                      hintText: 'Your email address',
+                      labelText: 'Email',
+                    ),
+                    onChanged: (value) {
+                      formData.email = value;
+                    },
+                  ),
+                  TextFormField(
+                    decoration: InputDecoration(
+                      filled: true,
+                      labelText: 'Password',
+                    ),
+                    obscureText: true,
+                    onChanged: (value) {
+                      formData.password = value;
+                    },
+                  ),
+                  FlatButton(
+                    child: Text('Sign in'),
+                    onPressed: () async {
+                      // Use a JSON encoded string to send
+                      var result = await widget.httpClient.post(
+                          'https://example.com/signin',
+                          body: json.encode(formData.toJson()),
+                          headers: {'content-type': 'application/json'});
+
+                      if (result.statusCode == 200) {
+                        _showDialog('Succesfully signed in.');
+                      } else if (result.statusCode == 401) {
+                        _showDialog('Unable to sign in.');
+                      } else {
+                        _showDialog('Something went wrong. Please try again.');
+                      }
+                    },
+                  ),
+                ].expand(
+                  (widget) => [
+                    widget,
+                    SizedBox(
+                      height: 24,
+                    )
+                  ],
+                )
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+
+  void _showDialog(String message) {
+    showDialog(
+      context: context,
+      child: AlertDialog(
+        title: Text(message),
+        actions: [
+          FlatButton(
+            child: Text('OK'),
+            onPressed: () => Navigator.of(context).pop(),
+          ),
+        ],
+      ),
+    );
+  }
+}