code: function(node, listener) {
        var observer = new MutationObserver(function(mutations) {
          listener.call(this, observer, mutations);
          observer.disconnect();
        }.bind(this));
        observer.observe(node, {childList: true, subtree: true});
      }
docs: Register a one-time callback when a child-list or sub-tree mutation
  occurs on node.
  
  For persistent callbacks, call onMutation from your listener.
  
  @method onMutation
  @param Node {Node} node Node to watch for mutations.
  @param Function {Function} listener Function to call on mutation. The function is invoked as `listener.call(this, observer, mutations);` where `observer` is the MutationObserver that triggered the notification, and `mutations` is the native mutation list.