json_schema_compiler: any, additionalProperties, functions on types

chrome/common/extensions/api/experimental.accessibility.json

 [
   {
     "namespace": "experimental.accessibility",
     "nodoc": true,
     "types": [
       {
         "id": "AccessibilityObject",
         "type": "object",
         "description": "Parent class for accessibility information about an object.",
         "properties": {
@@ skipping 131 matching lines @@
         "type": "function",
         "description": "Gets information about the currently focused control.",
         "parameters": [
           {
             "type": "function",
             "name": "callback",
             "parameters": [
               {
                 "name": "control",
                 "description": "Details of the currently focused control, or null if nothing is focused.",
-                "choices": [
-                  { "$ref": "AccessibilityObject" },
-                  { "type": "null" }
-                ]
+                "$ref": "AccessibilityObject",
+                "optional": true
               }
             ]
           }
         ]
       },
       {
         "name": "getAlertsForTab",
         "type": "function",
         "description": "Gets alerts being shown on the given tab.",
         "parameters": [
@@ skipping 98 matching lines @@
           {
             "$ref": "AccessibilityObject",
             "name": "menu",
             "description": "Information about the menu that was closed."
           }
         ]
       }
     ]
   }
 ]