code: 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;
  }
docs: 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