move listenOnce to core-transition

Author: Yvonne Yip

core-transition-css.html

@@ -62,16 +62,6 @@
       });
       node.classList.toggle(this.openedClass, opened);
       node.classList.toggle(this.closedClass, !opened);
-    },
-
-    // TODO(sorvell): worth promoting
-    listenOnce: function(node, event, fn, args) {
-      var self = this;
-      var listener = function() {
-        fn.apply(self, args);
-        node.removeEventListener(event, listener, false);
-      }
-      node.addEventListener(event, listener, false);
     }
 
   });

core-transition.html

@@ -28,8 +28,16 @@
 
       complete: function(node) {
         this.fire('core-transitionend', null, node);
-      }
+      },
 
+      listenOnce: function(node, event, fn, args) {
+        var self = this;
+        var listener = function() {
+          fn.apply(self, args);
+          node.removeEventListener(event, listener, false);
+        }
+        node.addEventListener(event, listener, false);
+      }
 
     });
   </script>