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

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  Constructor]
 interface AudioContext {
   // TODO(ager): Auto-generate this custom method when the info about retaining
   // typed arrays is in the IDL.
   [Custom] void decodeAudioData(ArrayBuffer audioData, AudioBufferCallback successCallback, AudioBufferCallback errorCallback);
 };
 
@@ skipping 472 matching lines @@
 };
 
 [Supplemental] interface SpeechInputEvent : Event {
     [Suppressed] readonly attribute SpeechInputResultList results;
 };
 
 [Supplemental] interface AudioBufferSourceNode : AudioSourceNode {
     [Suppressed] readonly attribute AudioParam gain;
 };
 
+[Supplemental]
+interface CSSStyleDeclaration {
+    [Supplemental] boolean __propertyQuery__(DOMString name);
+};
+
 Element implements GlobalEventHandlers;