Version 0.8.10.1

dart/tools/dom/dom.json

 {
   "ANGLEInstancedArrays": {
     "members": {
       "VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE": {
         "support_level": "untriaged"
       },
       "drawArraysInstancedANGLE": {
         "support_level": "untriaged"
       },
       "drawElementsInstancedANGLE": {
@@ skipping 315 matching lines @@
     },
     "support_level": "stable"
   },
   "BeforeLoadEvent": {
     "members": {
       "url": {}
     },
     "support_level": "experimental"
   },
   "BeforeUnloadEvent": {
-    "members": {},
-    "support_level": "untriaged"
+    "comment": "http://www.w3.org/html/wg/drafts/html/master/browsers.html#beforeunloadevent",
+    "members": {
+      "returnValue": {}
+    },
+    "support_level": "stable"
   },
   "BiquadFilterNode": {
     "comment": "https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#BiquadFilterNode-section",
     "members": {
       "ALLPASS": {},
       "BANDPASS": {},
       "HIGHPASS": {},
       "HIGHSHELF": {},
       "LOWPASS": {},
       "LOWSHELF": {},
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       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }