Add property to CssStyleDeclaration to detect if a CSS value is available.

tools/dom/dom.json

 {
   "ANGLEInstancedArrays": {
     "members": {
       "VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE": {
         "support_level": "untriaged"
       },
       "drawArraysInstancedANGLE": {
         "support_level": "untriaged"
       },
       "drawElementsInstancedANGLE": {
@@ skipping 588 matching lines @@
       "length": {}
     },
     "support_level": "stable"
   },
   "CSSStyleDeclaration": {
     "comment": "http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface",
     "members": {
       "__getter__": {
         "support_level": "untriaged"
       },
+      "__propertyQuery__": {
+        "support_level": "untriaged"
+      },
       "__setter__": {},
       "cssText": {},
       "getPropertyCSSValue": {
         "comment": "http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface",
         "dart_action": "suppress",
         "support_level": "deprecated"
       },
       "getPropertyPriority": {},
       "getPropertyValue": {},
       "item": {},
@@ skipping 11542 matching lines @@
   },
   "TimeoutHandler": {
     "comment": "http://www.whatwg.org/specs/web-apps/2007-10-26/multipage/section-timers.html#timeouthandler",
     "members": {
       "handleEvent": {}
     },
     "support_level": "stable"
   },
   "Timing": {
     "members": {
+      "__getter__": {
+        "support_level": "untriaged"
+      },
       "__setter__": {
         "support_level": "untriaged"
       },
       "delay": {
         "support_level": "untriaged"
       },
       "direction": {
         "support_level": "untriaged"
       },
       "easing": {
@@ skipping 2277 matching lines @@
       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }