docs: function watch(obj, _keyPath, m) {
      // can't watch length on Array - it is special...
      if (_keyPath === 'length' && Array.isArray(obj)) {
        return;
      }
  
      if (!_emberMetalPath_cache.isPath(_keyPath)) {
        _emberMetalWatch_key.watchKey(obj, _keyPath, m);
      } else {
        _emberMetalWatch_path.watchPath(obj, _keyPath, m);
      }
    }