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.g.dart

@@ -0,0 +1,23 @@
+// 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.
+
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'sign_in_http.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+FormData _$FormDataFromJson(Map<String, dynamic> json) {
+  return FormData(
+    email: json['email'] as String,
+    password: json['password'] as String,
+  );
+}
+
+Map<String, dynamic> _$FormDataToJson(FormData instance) => <String, dynamic>{
+      'email': instance.email,
+      'password': instance.password,
+    };