Stop generating MutationCallback right way.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  Constructor]
 interface AudioContext {};
 
 [Supplemental]
 interface Document {
   [Suppressed] DOMObject getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
   CanvasRenderingContext getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
@@ skipping 273 matching lines @@
 };
 
 [Supplemental, Callback] // Add missing Callback attribute.
 interface VoidCallback {
 };
 
 interface SVGNumber {
   [StrictTypeChecking, Custom] attribute float value;
 };
 
-// TODO(antonm): get rid of it to follow V8 bindings.
+// Keep it in to generate Dart code, C++ implementation is filterd out in generator.
 [
   Callback
 ] interface MutationCallback {
   [Custom] boolean handleEvent(in MutationRecordArray mutations, in MutationObserver observer);
 };
 
 [Supplemental,
   CustomConstructor,
   // Provide missing constructor signature.
   Constructor(MutationCallback callback)]
@@ skipping 32 matching lines @@
   [Suppressed] boolean send(in DOMString data) raises(DOMException);
 
   [Custom] void send(DOMString data) raises(DOMException);
   [Custom] void send(Blob data) raises(DOMException);
   [Custom] void send(ArrayBuffer data) raises(DOMException);
   [Custom] void send(ArrayBufferView data) raises(DOMException);
 };
 
 [Suppressed]
 interface Entity {};