function _isTransformer(obj) {
    return obj != null && typeof obj['@@transducer/step'] === 'function';
  }
Tests whether or not an object is an array.

@private
@param {*} val The object to test.
@return {Boolean} `true` if `val` is an array, `false` otherwise.
@example

     _isArray([]); //=> true
     _isArray(null); //=> false
     _isArray({}); //=> false