function getRadioButtons(parent) {
    return $mdUtil.iterator(parent[0].querySelectorAll('md-radio-button'), true);
  }
Coerce all child radio buttons into an array, then wrap them in an iterator.
@param parent {!JQLite}
@return {{add: add, next: (function()), last: (function(): any|null), previous: (function()), count: (function(): number), hasNext: (function(*=): Array.length|*|number|boolean), inRange: (function(*): boolean), remove: remove, contains: (function(*=): *|boolean), itemAt: (function(*=): any|null), findBy: (function(*, *): *[]), hasPrevious: (function(*=): Array.length|*|number|boolean), items: (function(): *[]), indexOf: (function(*=): number), first: (function(): any|null)}}