code: function(partialProps, callback) {
      // This is a deoptimized path. We optimize for always having an element.
      // This creates an extra internal element.
      var element = this._pendingElement || this._currentElement;
      this._pendingElement = ReactElement.cloneAndReplaceProps(
        element,
        assign({}, element.props, partialProps)
      );
      ReactUpdates.enqueueUpdate(this, callback);
    }
docs: Schedule a partial update to the props. Only used for internal testing.
  
  @param {object} partialProps Subset of the next props.
  @param {?function} callback Called after props are updated.
  @final
  @internal