function sortBreakpoints(breakpoints
/*: Breakpoints*/
)
/*: Array<Breakpoint>*/
{
  var keys
  /*: Array<string>*/
  = Object.keys(breakpoints);
  return keys.sort(function (a, b) {
    return breakpoints[a] - breakpoints[b];
  });
}
Given breakpoints, return an array of breakpoints sorted by width. This is usually
e.g. ['xxs', 'xs', 'sm', ...]

@param  {Object} breakpoints Key/value pair of breakpoint names to widths.
@return {Array}              Sorted breakpoints.