Add back CssCharsetRule for Safari support

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [DartSupplemental,
  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 405 matching lines @@
     //              is Uint8Array or sequence<unsigned long>.  For now suppress
     //              the new operation. Vijay suggestion, and I think this makes
     //              sense, change dart:html to have a send operation that takes
     //              a List<int> and have custom handler to call the correct
     //              method.  The dart:blink library needs to do something else
     //              postfix the name with send_sequence (?).
     [DartSuppress] void send(sequence<unsigned long> data, optional double timestamp);
 };
 
 Element implements GlobalEventHandlers;
+
+
+// This only exists in Safari, and is deprecated there. But include it anyway
+// because otherwise iterating CSSRules in Safari can yield unknown JS objects.
+interface CSSCharsetRule : CSSRule {
+    [MeasureAs=CSSCharsetRuleEncoding] attribute DOMString encoding;
+};