add json validation for plugins recipe

This CL adds json validation ofr plugins recipe.

Bug: https://github.com/flutter/flutter/issues/56143
Change-Id: I34b8a26803caf6414556a4cdd8dd9c9a7eaffdc8
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6281
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>

recipes/plugins/plugins.py

diff --git a/recipes/plugins/plugins.py b/recipes/plugins/plugins.py
index 9501857..d1522d9 100644
--- a/recipes/plugins/plugins.py
+++ b/recipes/plugins/plugins.py
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 DEPS = [
+    'flutter/json_util',
     'flutter/repo_util',
     'recipe_engine/context',
     'recipe_engine/path',
@@ -24,6 +25,9 @@
         url=api.properties.get('git_url'),
         ref=api.properties.get('git_ref')
     )
+  # Validates plugins builders json format.
+  api.json_util.validate_json(plugins_checkout_path.join('.ci'))
+
   env, env_prefixes = api.repo_util.flutter_environment(flutter_checkout_path)
   with api.context(env=env, env_prefixes=env_prefixes,
                    cwd=flutter_checkout_path):