function imageSize(img, ignoreErr) {
  utils.assertType(img, 'string', 'img');
  try {
    var img = new Image(this, img.string);
  } catch (err) {
    if (ignoreErr) {
      return [new nodes.Unit(0), new nodes.Unit(0)];
    } else {
      throw err;
    }
  }

  // Read size
  img.open();
  var size = img.size();
  img.close();

  // Return (w h)
  var expr = [];
  expr.push(new nodes.Unit(size[0], 'px'));
  expr.push(new nodes.Unit(size[1], 'px'));

  return expr;
}
Return the width and height of the given `img` path.

Examples:

   image-size('foo.png')
   // => 200px 100px

   image-size('foo.png')[0]
   // => 200px

   image-size('foo.png')[1]
   // => 100px

Can be used to test if the image exists,
using an optional argument set to `true`
(without this argument this function throws error
if there is no such image).

Example:

   image-size('nosuchimage.png', true)[0]
   // => 0

@param {String} img
@param {Boolean} ignoreErr
@return {Expression}
@api public