Support for callbacks on ScriptProcessNode.

sdk/lib/web_audio/dart2js/web_audio_dart2js.dart

Index: sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
diff --git a/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart b/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
index 890206960df9096fb37c24ccfabaebd6517c422d..c5b77923f503b9822a3094c9addaee4cfb031273 100644
--- a/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
+++ b/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
@@ -844,14 +844,18 @@ class ScriptProcessorNode extends AudioNode native "*ScriptProcessorNode" {
             controller.add(audioData);
           }
         };
-      JS('void', '#.onaudioprocess = #', this,
-          convertDartClosureToJS(callback, 1));
-
+      _setEventListener(callback);
       _eventStream = controller.stream;
     }
     return _eventStream;
   }
 
+    _setEventListener(callback) {
+      JS('void', '#.onaudioprocess = #', this,
+          convertDartClosureToJS(callback, 1));
+    }
+
+
   @DomName('ScriptProcessorNode.bufferSize')
   @DocsEditable
   final int bufferSize;