Supporting non-SSVs in CustomEvent.detail

tools/dom/dom.json

 {
   "ANGLEInstancedArrays": {
     "members": {
       "VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE": {
         "support_level": "untriaged"
       },
       "drawArraysInstancedANGLE": {
         "support_level": "untriaged"
       },
       "drawElementsInstancedANGLE": {
@@ skipping 1156 matching lines @@
   },
   "CustomElementConstructor": {
     "comment": "https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#dfn-custom-element-constructor-generation",
     "dart_action": "suppress",
     "members": {},
     "support_level": "experimental"
   },
   "CustomEvent": {
     "comment": "http://www.w3.org/TR/DOM-Level-3-Events/#interface-CustomEvent",
     "members": {
+      "_detail": {
+        "support_level": "untriaged"
+      },
       "detail": {},
       "initCustomEvent": {}
     },
     "support_level": "stable"
   },
   "DOMApplicationCache": {
     "members": {
       "CHECKING": {
         "support_level": "untriaged"
       },
@@ skipping 11705 matching lines @@
       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }