code: function ImageDimensions (assetPath, registeredAssets, callback) {
      sassUtils.assertType(assetPath, "string");
      sassUtils.assertType(registeredAssets, "map");
      var self = this;
      var getPath = self.checkImagePath(assetPath.getValue(), registeredAssets);
      getPath.then(function (success) {
        var imageDimensions = self.getDimensions(success);
        imageDimensions.then(function (dimensions) {
          callback(null, dimensions);
        }, function (err) {
          callback(err, null);
        });
      }, function (err) {
        callback(err, null);
      });
    }
docs: ImageDimensions constructor.
  @param {String}    assetPath         the file path from sass
  @param {Map}       registeredAssets  registered assets from sass
  @param {Function}  callback          callback function to deliver results