add json validation

Change-Id: I241fca15e124d48bd10dc0b371a2b89c044a8952
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5140
Commit-Queue: Keyong Han <keyonghan@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>