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/http/mock_client.dart

@@ -0,0 +1,31 @@
+// 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:mockito/mockito.dart';
+
+class MockClient extends Mock implements http.Client {
+  MockClient() {
+    when(post('https://example.com/signin', body: anyNamed('body')))
+        .thenAnswer((answering) {
+      var body = answering.namedArguments[Symbol('body')];
+
+      if (body != null && body is String) {
+        var decodedJson = json.decode(body);
+
+        if (decodedJson['email'] == 'root' &&
+            decodedJson['password'] == 'password') {
+          return Future.value(http.Response('', 200));
+        }
+      }
+
+      return Future.value(http.Response('', 401));
+    });
+
+    when(post('https://example.com/signout'))
+        .thenAnswer((_) => Future.value(http.Response('', 401)));
+  }
+}