code: function(expression, onAdd, onRemove, callbackContext) {
      if (!onAdd) onAdd = function(){};
      if (!onRemove) onRemove = function(){};
  
      var observer = this.createObserver(expression, this.createMemberObserverCallback.bind(this, onAdd, onRemove, callbackContext));
  
      observer.getChangeRecords = true;
      return observer;
    }
docs: Observe an expression and call `onAdd` and `onRemove` whenever a member is added/removed from the array or object.
  @param {String} expression The expression to observe
  @param {Function} onAdd The function which will be called when a member is added to the source
  @param {Function} onRemove The function which will be called when a member is removed from the source
  @return {Observer} The observer