add form_app (#463)

* add form_app

* revert unrelated changes

* use links in README

* remove unnecessary type annotation

* Remove GlobalKey for sign_in_http sample

* reword comment

* run package:copywriter

* use relative import

* fix quotes

* Add punctuation

* make sure setState() is used in form_widgets sample

* avoid first person pronoun

* store date state in parent widget in form_widgets.dart

experimental/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,
+    };