code: function tag(spotify, obj){
    if (obj === null || 'object' != typeof obj) return;
    Object.keys(obj).forEach(function(key){
      var val = obj[key];
      var type = typeof val;
      if ('object' == type) {
        if (Array.isArray(val)) {
          val.forEach(function (v) {
            tag(spotify, v);
          });
        } else {
          tag(spotify, val);
        }
      }
    });
    Object.defineProperty(obj, '_spotify', {
      value: spotify,
      enumerable: false,
      writable: true,
      configurable: true
    });
  }
docs: XXX: move to `util`?
  Attaches the `_spotify` property to each "object" in the passed in `obj`.
  
  @api private