We missed this because Flow wasn't being run properly on Travis but now that it is, it's clear that #7716 introduced a bunch of new Flow failures. Most of these shouldn't be too bad and I like to say "you break it you buy it" so this one is for you @Aweary :)

There were a couple other pre-existing Flow issues that @vjeux is going to fix, so mostly this should just be the TestRenderer issues.

}
    }
    nextChildren = flattenChildren(nextNestedChildrenElements, selfDebugID);
    ReactChildReconciler.updateChildren(
      prevChildren,
      nextChildren,
      mountImages,
      removedNodes,
      transaction,
      this,
      this._hostContainerInfo,
      context,
      selfDebugID
    );
    return nextChildren;
  },
  /**
   * Generates a "mount image" for each of the supplied children. In the case
   * of `ReactDOMComponent`, a mount image is a string of markup.
   *
   * @param {?object} nestedChildren Nested child maps.
   * @return {array} An array of mounted representations.
   * @internal
   */
  mountChildren: function(nestedChildren, transaction, context) {
    var children = this._reconcilerInstantiateChildren(
      nestedChildren, transaction, context
    );
    this._renderedChildren = children;
    var mountImages = [];