Update jsonexample (#72)

- use latest dependencies
- update build.yaml for removed option
- rebuilt generated code

jsonexample/build.yaml

@@ -14,13 +14,3 @@ targets:
       json_serializable|json_serializable:
         generate_for:
         - lib/json_serializable/*.dart
-        options:
-          header: |
-            // Copyright 2018 The Chromium Authors. 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
-
-
-