function onlyProgressBar(props, propName, componentName) {
  var children = props[propName];

  if (!children) {
    return null;
  }

  var error = null;
  external_root_React_commonjs2_react_commonjs_react_amd_react_default.a.Children.forEach(children, function (child) {
    if (error) {
      return;
    }
    /**
     * Compare types in a way that works with libraries that patch and proxy
     * components like react-hot-loader.
     *
     * see https://github.com/gaearon/react-hot-loader#checking-element-types
     */


    var element = external_root_React_commonjs2_react_commonjs_react_amd_react_default.a.createElement(DecoratedProgressBar, {
      __source: {
        fileName: ProgressBar_jsxFileName,
        lineNumber: 33
      },
      __self: this
    });
    if (child.type === element.type) return;
    var childIdentifier = external_root_React_commonjs2_react_commonjs_react_amd_react_default.a.isValidElement(child) ? child.type.displayName || child.type.name || child.type : child;
    error = new Error("Children of " + componentName + " can contain only ProgressBar " + ("components. Found " + childIdentifier + "."));
  });
  return error;
}
Validate that children, if any, are instances of `<ProgressBar>`.