code: function (root, tagName) {
      var all = ReactTestUtils.scryRenderedDOMComponentsWithTag(root, tagName);
      if (all.length !== 1) {
        throw new Error('Did not find exactly one match (found: ' + all.length + ') ' + 'for tag:' + tagName);
      }
      return all[0];
    }
docs: Like scryRenderedDOMComponentsWithTag but expects there to be one result,
  and returns that one result, or throws exception if there is any other
  number of matches besides one.
  @return {!ReactDOMComponent} The one match.