function instantiateChild(childInstances, child, name, selfDebugID) {
  // We found a component instance.
  var keyUnique = childInstances[name] === undefined;
  if (process.env.NODE_ENV !== 'production') {
    if (!ReactComponentTreeHook) {
      ReactComponentTreeHook = __webpack_require__(11);
    }
    if (!keyUnique) {
      process.env.NODE_ENV !== 'production' ? warning(false, 'flattenChildren(...): Encountered two children with the same key, ' + '`%s`. Child keys must be unique; when two children share a key, only ' + 'the first child will be used.%s', KeyEscapeUtils.unescape(name), ReactComponentTreeHook.getStackAddendumByID(selfDebugID)) : void 0;
    }
  }
  if (child != null && keyUnique) {
    childInstances[name] = instantiateReactComponent(child, true);
  }
}
Updates the rendered children and returns a new set of children.

@param {?object} prevChildren Previously initialized set of children.
@param {?object} nextChildren Flat child element maps.
@param {ReactReconcileTransaction} transaction
@param {object} context
@return {?object} A new set of child instances.
@internal